FIG.  1. — Construction  of  Peck's  Run  Sewer,  Baltimore,  Maryland. 


Frontispiece. 


SEWERAGE*0*** 

AND 

SEWAGE  TREATMENT 


BY 

HAROLD  E.   BABBITT,  M.S. 

Assistant  Professor,  Municipal  and  Sanitary  Engineering, 

University    of   Illinois;    Associate    Member 

American  Society  of  Civil  Engineers 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 

LONDON:    CHAPMAN   &   HALL,  LIMITED 

1922 


Copyright,  1922,  by 
HAROLD  E.  BABBITT,  M.S. 


PRESS    OF 

BRAUNWOHTH     IL     CO. 

BOOK    MANUFACTURERS 

BROOKLYN.    N.    Y. 


PREFACE 


THIS  book  is  a  development  of  class-room  and  lecture  notes 
prepared  by  the  author  for  use  in  his  classes  at  the  University 
of  Illinois.  He  has  found  such  notes  necessary,  since  among 
the  many  books  dealing  with  sewerage  and  sewage  treatment 
he  has  found  none  suitable  as  a  text-book  designed  to  cover  the 
entire  subject.  The  need  for  a  single  book  of  the  character 
described  has  been  expressed  by  engineers  in  practice,  and  by 
students  and  teachers  for  use  in  the  class-room.  This  book 
has  been  prepared  to  meet  both  these  needs.  It  is  hoped  that 
the  searching  questions  propounded  by  students  in  using  the 
original  notes,  and  the  suggestions  and  criticisms  of  engineers 
and  teachers  who  have  read  the  manuscript,  have  resulted  in  a 
text  which  can  be  readily  understood. 

The  ground  covered  includes  an  exposition  of  the  princ:ples 
and  methods  for  the  designing,  construction  and  maintenance  of 
sewerage  works,  and  also  of  the  treatment  of  sewage.  In  covering 
so  wide  a  field  the  author  has  deemed  it  necessary  to  include  some 
chapters  which  might  equally  well  appear  in  works  on  other 
branches  of  engineering,  such  as  the  chapter  on  Pumps  and 
Pumping  Stations.  Special  stress  has  been  laid  on  che  funda- 
mentals of  the  subject  rather  than  the  details  of  practice,  although 
illustrations  have  been  drawn  freely  from  practical  work.  The 
quotation  of  expert  opinions  which  may  be  in  controversy,  or  the 
citation  of  examples  of  different  methods  of  accomplishing  the 
same  thing,  has  been  avoided  when  possible  in  order  to  simplify 
explanations  and  to  avoid  confusing  the  beginner. 

The  work  is  to  some  extent  a  compilation  of  notes  and  quota- 
tions which  have  been  collected  by  the  author  during  years 
of  study  and  teaching  the  subject.  Credit  has  been  given 
wherever  due,  and  at  the  same  time  references  have  pointed  out 
the  original  sources  whenever  possible.  These  references,  which 


481200 


vi  PREFACE 

have  been  supplemented  by  brief  bibliographies  at  the  end  of 
certain  chapters,  will  be  useful  to  the  student  and  engineer  inter- 
ested in  further  study.  Occasionally  the  original  reference  has 
been  lost  or  the  phraseology  of  a  quotation  has  been  so  altered 
by  class-room  use,  as  to  make  it  impossible  to  trace  the  original 
source,  so  that  in  some  few  instances  full  credit  may  be  lacking. 

The  author  is  indebted  to  many  of  his  friends  for  their  criti- 
cisms and  suggestions  in  the  preparation  of  the  manuscript; 
but  he  desires  particularly  to  acknowledge  the  assistance  of 
Professor  A.  N.  Talbot,  Professor  of  Municipal  and  Sanitary 
Engineering  at  the  University  of  Illinois,  and  of  Professor  M.  L. 
Enger,  Professor  of  Mechanics  and  Hydraulics  at  the  University 
of  Illinois,  in  the  entire  work;  also  that  of  Mr.  T.  D.  Pitts,  Prin- 
cipal Assistant  Engineer  of  the  Baltimore  Sewerage  Commis- 
sion during  the  construction  of  the  Baltimore  sewers,  for  his 
suggestions  on  the  first  half  of  the  book;  and  to  Mr.  Paul  Hansen, 
consulting  engineer,  of  Chicago,  and  to  Mr.  Langdon  Pearse, 
Sanitary  Engineer  of  the  Sanitary  District  of  Chicago,  for 
their  help  on  the  section  covering  the  treatment  of  sewage;  and 
to  Professor  Edward  Bartow,  Professor  of  Chemistry  at  the 
University  of  Iowa,  for  his  review  of  the  chapter  on  Activated 
Sludge;  in  general  his  thanks  are  due  to  all  others  who  have 
furnished  suggestions,  illustrations,  or  quotations,  acknowledg- 
ments of  which  have  been  included  in  the  text. 

H.  E.  B. 
URBANA,  ILLINOIS,   1922. 


TABLE   OF  CONTENTS 


CHAPTER  I 

INTRODUCTION 

PAGES 

1.  Sewerage  and  the  Sanitary  Engineer.  2.  Historical.  3.  Methods 
of  Collection.  4.  Methods  of  Disposal.  5.  Methods  of  Treat- 
ment. 6.  Definitions 1-8 


CHAPTER  II 
WORK  PRELIMINARY  TO  DESIGN 

7.  Division  of  Work.  8.  Preliminary.  9.  Estimate  of  cost.  METH- 
ODS OF  FINANCING.  10.  Bond  Issues.  11.  Special  Assessment. 
12.  General  Taxation.  13.  Private  Capital.  PRELIMINARY 
WORK.  14.  Preparing  for  Design.  15.  Underground  Surveys. 
16,  Borings 9-23 

CHAPTER  III 
QUANTITY  OF  SEWAGE 

17.  Dry  Weather  Flow.  18.  Methods  for  Predicting  Population. 
19.  Extent  of  Prediction.  20.  Sources  of  Information  on 
Population.  21.  Density  of  Population.  22.  Changes  in  Area. 
23.  Relation  between  Population  and  Sewage  Flow.  24.  Char- 
acter of  District.  25.  Fluctuations  in  Rate  of  Sewage  Flow. 
26.  Effect  of  Ground  Water.  27.  Resume  of  Method  for 
Determination  of  Quantity  of  Dry-weather  Sewage.  QUANTITY 
OF  STORM  WATER.  28.  The  Rational  Method.  29.  Rate  of 
Rainfall.  30.  Time  of  Concentration.  31.  Character  of  Sur- 
face. 32.  Empirical  Formulas.  33.  Extent  and  Intensity  of 

Storms 24-50 

vii 


viii  CONTENTS 

CHAPTER  IV 

HYDRAULICS  OF  SEWERS 

PAGES 

34.  Principles.  35.  Formulas.  36.  Solution  of  Formulas.  37.  Use 
of  Diagrams.  38.  Flow  in  Circular  Pipes  Partly  Full.  39.  Sec- 
tions Other  than  Circular.  40.  Non-Uniform  Flow 51-77 

CHAPTER  V 

DESIGN  OF  SEWERAGE  SYSTEMS 

41.  The  Plan.  42.  Preliminary  Map.  43.  Layout  of  the  Separate 
System.  44.  Location  and  Numbering  of  Manholes.  45. 
Drainage  Areas.  46.  Quantity  of  Sewage.  47.  Surface  Profile. 
48.  Slope  and  Diameter  of  Sewers.  49.  The  Sewer  Profile. 
DESIGN  OF  A  STORM-WATER  SEWER  SYSTEM.  50.  Planning  the 
System.  51.  Location  of  Street  Inlets.  52.  Drainage  Areas. 
53.  Computation  of  Flood  Flow  by  Me  Math  Formula.  54. 
Computation  of  Flood  Flow  by  Rational  Method 78-98 


CHAPTER  VI 

APPURTENANCES 

55.  General.  56.  Manholes.  57.  Lampholes.  58.  Street  Inlets. 
59.  Catch-basins.  60.  Grease  Traps.  61.  Flush-tanks.  62. 
Siphons.  63.  Regulators.  64.  Junctions.  65.  Outlets.  66. 
Foundations.  67.  Underdrains. .  99-126 


CHAPTER  VII 
PUMPS  AND  PUMPING  STATIONS 

68.  Need.  69.  Reliability.  70.  Equipment.  71.  The  Building. 
72.  Capacity  of  Pumps.  73  Capacity  of  Receiving  Well.  74. 
Types  of  Pumping  Machinery.  75.  Sizes  and  Descriptions  of 
Pumps.  76.  Definitions  of  Duties  and  Efficiency.  77.  Details 
of  Centrifugal  Pumps.  78.  Centrifugal  Pump  Characteristics. 
79.  Setting  of  Centrifugal  Pumps.  80.  Steam  Pumps  and 
Pumping  Engines.  81.  Steam  Turbines.  82.  Steam  Boilers. 
83.  Air  Ejectors.  84.  Electric  Motors.  85.  Internal  Com- 


CONTENTS  ix 

PAGES 

bustion  Engines.  86.  Selection  of  Pumping  Machinery.  87. 
Costs  of  Pumping  Machinery.  88.  Cost  Comparisons  of  Dif- 
ferent Designs.  89.  Number  and  Capacity  of  Pumping  Units.  127-163 

CHAPTER  VIII 

MATERIALS  FOR  SEWERS 

90.  Materials.  91.  Vitrified  Clay  Pipe.  92.  Cement  and  Concrete 
Pipe.  93.  Proportioning  of  Concrete.  94.  Waterproofing  of 
Concrete.  95.  Mixing  and  Placing  Concrete.  96.  Sewer  Brick. 
97.  Vitrified  Clay  Sewer  Block.  98.  Cast  Iron,  Steel,  and  Wood.  164-193 

CHAPTER  IX 
DESIGN  OF  THE  SEWER  RING 

99.  Stresses  in  Buried  Pipe.  100.  Design  of  Steel  Pipe.  101. 
Design  of  Wood  Stave  Pipe.  102.  External  Loads  on  Buried 
Pipe.  103.  Stresses  in  Circular  Ring.  104.  Analysis  of  Sewer 
Arches.  105.  Reinforced  Concrete  Sewer  Design 194-210 

CHAPTER  X 
CONTRACTS  AND  SPECIFICATIONS 

106.  Importance  of  the  Subject.  107.  Scope  of  the  Subject.  108. 
Types  of  Contracts.  109.  The  Agreement.  110.  The  Advertise- 
ment. 111.  Information  and  Instructions  for  Bidders.  112. 
Proposal.  113.  General  Specifications.  114.  Technical  Specifi- 
cations. 115.  Special  Specifications.  116.  The  Contract. 
117.  The  Bond 211-232 

CHAPTER  XI 
CONSTRUCTION 

118.  Elements.  WORK  OF  THE  ENGINEER.  119.  Duties.  120. 
Inspection.  121.  Interpretation  of  Contract.  122.  Unex- 
pected Situations.  123.  Cost  Data  and  Estimates.  124. 
Progress  Reports.  125.  Records.  EXCAVATION.  126.  Speci- 
fications. 127.  Hand  Excavation.  128.  Machine  Excavation. 


X  CONTENTS 

PAGES 

129.  Types  of  Machines.  130.  Continuous  Bucket  Exca- 
vators. 131.  Cableway  and  Trestle  Excavators.  132.  Tower 
Cableways.  133.  Steam  Shovels.  134.  Drag  Line  and  Bucket 
Excavators.  135.  Excavation  in  Quicksand.  136.  Pumping 
and  Drainage.  137.  Trench  Pump.  138.  Diaphragm  Pump. 
139.  Jet  Pump.  140.  Steam  Vacuum  Pumps.  141.  Centrif- 
ugal and  Reciprocating  Pumps.  142.  Well  Points.  143.  Rock 
Excavation.  144.  Power  Drilling.  145.  Steam  or  Air  for  Power. 
146.  Depth  of  Drill  Hole.  147.  Diameter  of  Drill  Hole. 

148.  Spacing     of     Drill     Holes.     SHEETING     AND     BRACING. 

149.  Purposes  and  Types.     150.  Stay  Bracing.     151.  Skeleton 
Sheeting.      152.  Poling    Boards.      153.  Box    Sheeting.      154. 
Vertical  Sheeting.     155.  Pulling  Wood  Sheeting.     156.  Earth 
Pressures.     157.  Design  of  Sheeting  and  Bracing.     158.  Steel 
Sheet  Piling.     LINE  AND  GRADE.     159.  Locating  the  Trench. 
160.  Final    Line    and    Grade.      161.  Transferring    Grade    and 
Line  to  the  Pipe.     162.  Line  and  Grade  in  Tunnel.     TUN- 
NELLING.     163.  Depth.      164.  Shafts.      165.  Timbering.      166. 
Shields.      167.  Tunnel    Machines.      168.  Rock   Tunnels.     169. 
Ventilation.    170.  Compressed  Air.    EXPLOSIVES  AND  BLASTING. 
171.  Requirements.      172.  Types    of    Explosives.      173.  Per- 
missible Explosives.     174.  Strength.     175.  Fuses    and    Deto- 
nators.    176.  Care  in  Handling.     177.  Priming.   Loading,  and 
Firing.    178.  Quantity  of  Explosive.    PIPE  SEWERS.    179.  The 
Trench  Bottom.     180.  Laying  Pipe.     181.  Joints.     182.  Labor 
and  Progress.     BRICK  AND  BLOCK  SEWERS.     183.  The  Invert. 
184.  The     Arch.       185.  Block     Sewers.       186.  Organization. 
187.  Rate  of  Progress.    CONCRETE  SEWERS.    188.  Construction 
in  Open  Cut.     189.  Construction  in  Tunnels.     190.  Materials 
for  Forms.    191.  Design  of  Forms.    192.  Wooden  Forms.     193. 
Steel-lined   Wooden   Forms.      194.  Steel   Forms.      195.  Rein- 
forcement. 196.  Cost  of  Concrete  Sewers.    BACKFILLING.    197. 
Method 233-331 

CHAPTER  XII 

MAINTENANCE  OF  SEWERS 

198.  Work  Involved.  199.  Causes  of  Troubles.  200.  Inspection. 
201.  Repairs.  202.  Cleaning  of  Sewers.  203.  Flushing  Sewers. 
204.  Cleaning  Catch-basins.  205.  Protection  of  Sewers.  206. 
Explosions  in  Sewers,  207.  Valuation  of  Sewers 332-351 

CHAPTER  XIII 

COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

208.  Physical  Characteristics.  209.  Chemical  Composition.  210. 
Significance  of  Chemical  Constituents.  211.  Sewage  Bacteria. 


CONTENTS  xi 


PAGES 

212.  Organic  Life  in  Sewage.  213.  Decomposition  of  Sewage. 
214.  The  Nitrogen  Cycle.  215.  Plankton  and  Macroscopic 
Organisms.  216.  Variations  in  the  Quality  of  Sewage.  217. 
Sewage  Disposal.  218.  Methods  of  Sewage  Treatment 352-371 


CHAPTER  XIV 
DISPOSAL  BY  DILUTION 

219.  Definition.  220.  Conditions  Required  for  Success.  221.  Self- 
purification  of  Running  Streams.  222.  Self-purification  of 
Lakes.  223.  Dilution  in  Salt  Water.  224.  Quantity  of  Diluting 
Water  Needed.  225.  Governmental  Control.  226.  Prelimi- 
nary Treatment.  227.  Preliminary  Investigations 372-382 

CHAPTER  XV 

SCREENING  AND  SEDIMENTATION 

228.  Purpose.  229.  Types  of  Screens.  230.  Sizes  of  Openings. 
231.  Design  of  Fixed  and  Movable  Screens.  PLAIN  SEDIMEN- 
TATION. 232.  Theory  of  Sedimentation.  233.  Types  of  Sedi- 
mentation Basins.  234.  Limiting  Velocities.  235.  Quantity 
and  Character  of  Grit.  236.  Dimensions  of  Grit  Chambers. 
237.  Existing  Grit  Chambers.  238.  Number  of  Grit  Chambers. 
239.  Quantity  and  Characteristics  of  Sludge  from  Plain  Sedi- 
mentation. 240.  Dimensions  of  Sedimentation  Basins.  CHEM- 
ICAL PRECIPITATION.  241.  The  Process.  242.  Chemicals. 
243.  Preparation  and  Addition  of  Chemicals.  244.  Results. . . .  383-409 

CHAPTER  XVI 
SEPTICIZATION 

245.  The  Process.  246.  The  Septic  Tank.  247.  Results  of  Septic 
Action.  248.  Design  of  Septic  Tanks.  249.  ImhorT  Tanks. 
250.  Design  of  ImhorT  Tanks.  251.  Imhoff  Tank  Results. 
252.  Status  of  Imhoff  Tanks.  253.  Operation  of  ImhorT  Tanks. 
254.  Other  Tanks 410-430 

CHAPTER  XVII 
FILTRATION  AND  IRRIGATION 

255.  Theory.  256.  The  Contact  Bed.  257.  The  Trickling  Filter. 
258.  Intermittent  Sand  Filter.  259.  Cost  of  Filtration.  IRRI- 
GATION. 260.  The  Process.  261.  Status.  262.  Preparation 
and  Operation.  263.  Sanitary  Aspects.  264.  The  Crop 431-464 


xii  CONTENTS 

CHAPTER  XVIII 

ACTIVATED  SLUDGE 

PAGES 

265.  The  Process.  266.  Composition.  267.  Advantages  and 
Disadvantages.  268.  Historical.  269.  Aeration  Tank.  270. 
Sedimentation  Tank.  271.  Reaeration  Tank.  272.  Air  Dis- 
tribution. 273.  Obtaining  Activated  Sludge.  274.  Cost 465-479 

CHAPTER  XIX 
ACID  PRECIPITATION,  LIME  AND  ELECTRICITY,  AND  DISINFECTION 

275.  The  Miles  Acid  Process.     ELECTROLYTIC  TREATMENT.     276. 

The  Process.    DISINFECTION.    277.  Disinfection  of  Sewage.  .  . .  482-493 

CHAPTER  XX 
SLUDGE 

278.  Methods  of  Disposal.    279.  Lagooning.    280.  Dilution.    281.  « 

Burial.    282.  Drying 495-505 

CHAPTER  XXI 

AUTOMATIC  DOSING  DEVICES 

283.  Types."1  284.  Operation.  285.  Three  Alternating  Siphons. 
286.  Four  or  More  Alternating  Siphons.  287.  Timed  Siphons. 
288.  Multiple  Alternating  and  Timed  Siphons 506-512 


SEWERAGE  AND  SEWAGE  TREATMENT 


CHAPTER  I 
INTRODUCTION 

1.  Sewerage  and  the  Sanitary  Engineer. — Present  day  concep- 
tions of  sanitation  are  based  on  the  scientific  discoveries  which 
have  resulted  so  much  in  the  increased  comfort  and  safety  of 
human  life  during  the  past  century,  in  the  increase  of  our  material 
possessions,  and  the  extent  of  our  knowledge.  The  danger  to 
health  in  the  accumulation  of  filth,  the  spreading  of  disease  by 
various  agents,  the  germ  theory  of  disease,  and  other  important 
principles  of  sanitation  can  be  counted  among  the  more  recent 
scientific  discoveries  and  pronouncements.  Experience  has  shown, 
and  continues  to  show,  that  the  increase  of  population  may  be 
inhibited  by  accumulations  of  human  waste  in  populous  districts. 
The  removal  of  these  wastes  is  therefore  essential  to  the  existence 
of  our  modern  cities. 

The  greatest  need  of  a  modern  city  is  its  water  supply.  With- 
out it  city  life  would  be  impossible.  The  next  most  important 
need  is  the  removal  of  waste  matters,  particularly  wastes  con- 
taining human  excreta  or  the  germs  of  disease.  To  exist  without 
street  lights,  pavements,  street  cars,  telephones,  and  the  many 
other  attributes  of  modern  city  life  might  be  possible,  although 
uncomfortable.  To  exist  in  a  large  city  without  either  water  or 
sewerage  would  be  impossible.  The  service  rendered  by  the  sani- 
tary engineer  to  the  large  municipality  is  indispensable.  In 
addition  to  the  service  necessary  to  the  maintenance  of  life  in 
large  cities,  the  sanitary  engineer  serves  the  smaller  city,  the 
rural  community,  the  isolated  institution,  and  the  private  estate 
with  sanitary  conveniences  which  make  possible  comfortable 


2  INTRODUCTION 

existence  in  them,  and  which  are  frequently  considered  as  of 
paramount  necessity.  Training  for  service  in  municipal  sanita- 
tion is  training  for  a  service  which  has  a  more  direct  beneficial 
effect  on  humanity  than  any  other  engineering  work,  or  any 
other  profession.  W.  P.  Gerhard  states: 

A  Sanitary  Engineer  is  an  engineer  who  carries  out  those 
works  of  civil  engineering  which  have  for  their  object: 

(a)  The  promotion  of  the  public  and  individual  health; 

(6)  The  remedying  of  insanitary  conditions; 

(c)  The  prevention  of  epidemic  diseases. 

A  well-educated  sanitary  engineer  should  have  a 
thorough  knowledge  of  general  civil  engineering,  of  archi- 
tecture, and  of  sanitary  science.  The  practice  of  the  sani- 
tary engineer  embraces  water  supply,  sewerage,  and 
sewage  and  garbage  disposal  for  cities  and  for  single  build- 
ings; the  prevention  of  river  pollution,  the  improvement 
of  polluted  water  supplies;  street  paving  and  street  clean- 
ing, municipal  sanitation,  city  improvement  plans,  the 
laying-out  of  cities,  the  preparation  of  sanitary  surveys, 
the  regulation  of  noxious  trades,  disinfection,  cremation, 
and  the  sanitation  of  buildings. 

The  need  of  the  work  of  the  sanitary  engineer  in  the  provision 
of  sewers  and  drains  is  thrust  upon  us  in  our  daily  experience  by 
the  clogging  of  sewers,  the  flooding  of  streets  by  heavy  rains, 
filthy  conditions  in  unsewered  districts,  increased  values  of  prop- 
erty and  improved  conditions  of  living  in  sewered  districts,  and 
in  many  other  ways.  The  increasing  demand  for  sewerage  and 
the  amount  of  money  expended  on  sewer  construction  is  indicated 
by  the  information  given  in  Table  I. 

2.  Historical. — An  ordinance  passed  by  the  Roman  Senate  in 
the  name  of  the  Emperor  about  A.D.  80,  states: 

I  desire  that  nobody  shall  conduct  away  any  excess 
water  without  having  received  my  permission  or  that  of  my 
representatives;  for  it  is  necessary  that  a  part  of  the  supply 
flowing  from  the  delivery  tanks  shall  be  utilized  not  only  for 
cleaning  our  city,  but  also  for  flushing  the  sewers.1 

Neither  the  sewers  mentioned  nor  the  distributing  pipes  of 
the  public  water  supply  were  connected  to  individual  residences. 
The  contributions  to  the  sewers  came  from  the  ground  and  the 
street  surface.  The  streets  were  the  receptacles  of  liquid  and 

1  Frontinus  and  the  Water  Supply  of  Rome,  p.  81,  by  Clemens  Herschel. 


HISTORICAL 


3 


solid  wastes  and  were  often  little  more  than  open  sewers.  A 
promenade  after  dark  in  an  ancient,  medieval,  or  early  modern 
city  was  accompanied  not  only  by  the  underfoot  dangers  of  an 
uneven  pavement  or  an  encounter  with  a  footpad,  but  with  the 
overhead  danger  from  the  emptying  of  slops  into  the  streets  from 
the  upper  windows.  Sewers  were  used  for  the  collection  of  sur- 
face water;  the  discharge  of  fecal  matter  into  them  was  pro- 
hibited. The  problem  of  the  collection  of  sewage  remained 
unsolved  until  the  Nineteenth  Century. 

TABLE  1 

POPULATION  TRIBUTARY  TO  SEWERAGE  SYSTEMS 


1905* 

1915f 

1920  1 

Population  discharging  raw  sewage  into 
the  sea  or  tidal  estuaries  
Population  discharging  raw  sewage  into 
inland  streams  or  lakes  
Population  connected  to  systems  where 
sewage  is  treated  in  some  way  

6,500,000 
20,400,000 
1,100,000 

8,500,000 
26,400,000 
6,900,000 

Population  connected  with  sewerage  sys- 
tems 

28,000,000 

41,800,000 

46,300  000 

*  Estimated  by  G.  W.  Fuller,  Trans.  Am.  Society  of  Civil  Engineers,  Vol.  44,  1905, 
p.  148.  The  total  population  connected  with  sewerage  systems  was  assumed  to  be  the 
total  population  in  the  United  States  in  cities  over  4000  in  population. 

t  Estimated  by  Metcalf  and  Eddy,  American  Sewerage  Practice,  Vol.  Ill,  p.  240. 

t  Computed  from  report  of  the  United  States  Census,  1920,  on  the  same  basis  as 
Fuller's  estimate  for  1905. 

The  development  of  the  London  sewers  was  commenced 
early  in  the  Nineteenth  Century.  The  sewerage  system  of  Ham- 
burg, Germany,  was  laid  out  in  1842  by  Lindley,  an  English 
engineer  who  with  other  English  engineers  performed  similar 
work  in  other  German  cities  because  of  their  earlier  experience 
in  English  communities.  Berlin's  present  system  dates  from  1860. 
The  construction  of  storm  water  drains  in  Paris  dates  from  1663.1 
They  were  intended  only  as  street  drains  but  are  now  included  in 
the  comprehensive  system  of  the  city.  The  first  comprehensive 
sewerage  system  in  the  United  States  was  designed  by  E.  S. 
Chesbrough  for  the  City  of  Chicago  in  1855.  Previous  to  this 
1  Cosgrove,  History  of  Sanitation. 


4  INTRODUCTION 

time  sewers  had  been  installed  in  an  indifferent  manner  and  with- 
out definite  plan.  The  installation  of  a  comprehensive  sewerage 
system  in  Baltimore  in  1915  marks  the  completion  of  installation 
of  sewerage  systems  in  all  large  American  cities. 

In  the  early  days  of  sewerage  design  it  was  considered  unsafe 
to  discharge  domestic  wastes  into  the  sewers  as  the  concentration  of 
so  much  sewage  was  expected  to  create  great  nuisances  and 
dangers  to  health.  That  the  fear  that  the  concentration  of  large 
quantities  of  sewage  would  create  a  nuisance  was  not  ill  founded 
is  proven  by  the  conditions  on  the  Thames  at  London  in  1858-59. 
Dr.  Budd  states:  l 

For  the  first  time  in  the  history  of  man,  the  sewage 
of  nearly  three  millions  of  people  had  been  brought  to 
seethe  and  ferment  under  a  burning  sun  in  one  vast  open 
cloaca  lying  in  their  midst. 

The  result  we  all  know.  Stench  so  foul  we  may  wrell 
believe  had  never  before  ascended  to  pollute  this  lower 
air.  Never  before  at  least  had  a  stink  risen  to  the  height 
of  an  historic  event.  .  .  For  months  together  the  topic 
almost  monopolized  the  public  prints  .  .  .  'India  is  in 
revolt  and  the  Thames  stinks'  were  the  two  great  facts 
coupled  together  by  a  distinguished  foreign  writer,  to  mark 
the  climax  of  a  national  humiliation.2 

The  problem  of  sewage  disposal  followed  the  more  or  less 
successful  solutions  of  the  problem  of  sewage  collection.  In 
England  the  British  Royal  Commission  on  Sewage  Disposal  was 
appointed  in  1857  and  issued  its  first  report  in  1865.  The  first 
studies  in  the  United  States  were  started  in  1887  by  the  establish- 
ment of  an  experiment  station  at  Lawrence,  Massachusetts,  where 
valuable  work  has  been  done.  The  station  is  under  the  State 
Board  of  Health,  which  issued  its  first  report  containing  the 
results  of  the  work  at  the  station,  in  1890. 

Various  methods  of  sewage  treatment  preparatory  to  disposal 
have  been  devised  from  time  to  time.  Some  have  fallen  into 
disuse,  such  as  the  A.  B.  C.  (alum,  blood  and  clay)  process,  and 
others  have  taken  a  permanent  place,  such  as  the  septic  tank. 
The  unsolved  problems  of  sewage  collection,  and  the  number  of 

1  Sedgwick:  Sanitary  Science  and  Public  Health. 

2  No  detrimental  effect  on  the  public  health  was  noted  as  a  result  of  this 
condition  however.     It  has  never  been  conclusively  proven  that  such  nuisances 
are  detrimental  to  the  public  health. 


METHODS  OF  COLLECTION  5 

persons  still  unserved  by  sewerage  and  sewage  disposal  opens  a 
wide  field  to  the  study  and  construction  of  sewerage  works. 

3.  Methods  of  Collection. — The  method  of  collection  which 
involves  the  removal  of  night  soil  from  a  privy  vault,  the  pail 
system  which  involves  the  collection  of  buckets  of  human  excreta 
from  closets  and  homes,  indoor  chemical  closets,  and  other  make- 
shift methods  of  collection  are  of  extreme  importance  where  no 
sewers  exist,  but  they  are  not  properly  considered  as  sewerage 
systems  or  sewerage  works.     These  methods  of  collection  are 
generally   confined   to   rural   districts  and   to   outlying   parts  of 
urban  communities.     They  require  constant  attention  for  their 
proper  conduct  and  little  skill  for  their  installation,  the  principal 
requirements  being  to  make  the  receptacles  fly-proof. 

The  pneumatic  system  was  introduced  by  Liernur,  a  Dutch 
engineer.1  It  is  used  in  parts  of  a  few  cities  in  Europe,  but  it  is 
not  capable  of  use  on  a  large  scale.  It  consists  of  a  system  of 
air-tight  pipes,  connecting  water  closets,  kitchen  sinks,  etc.,  with 
a  central  pumping  station  at  which  an  air-tight  tank  is  provided 
from  which  the  air  is  partly  exhausted.  As  little  water  as  possible 
is  allowed  to  mix  with  the  fecal  matter  and  other  wastes  in  order 
not  to  overtax  the  system.  Solid  and  liquid  wastes  are  drawn 
to  the  central  station  when  the  waste  valve  on  the  plumbing 
fixture  is  opened. 

The  collection  of  sewage  in  a  system  of  pipes  through  which  it 
is  conducted  by  the  buoyant  effect  and  scouring  velocity  of  water 
is  known  as  the  water  carriage  system.  This  is  the  only  method 
of  sewage  collection  in  general  use  in  urban  communities.  In 
this  system  solid  and  liquid  wastes  are  so  highly  diluted  with 
water  as  either  to  float  or  to  be  suspended  therein.  The  mixture 
resulting  from  this  high  dilution  follows  the  laws  of  hydraulics  as 
applied  to  pure  water,  or  water  containing  suspended  matter. 
It  will  flow  freely  through  properly  designed  conduits  and  will 
concentrate  the  sewage  wastes  at  the  point  of  ultimate  disposal. 

4.  Methods  of  Disposal. — Sewage  is  disposed  of  by  dilution  in 
water,  by  treatment  on  land,  or  occasionally  by  discharging  it 
into  channels  that  contain  no  diluting  water.     Some  form  of  treat- 
ment to  prepare  sewage  for  ultimate  disposal  is  frequently  neces- 
sary and  will  undoubtedly  be  required  in  a  comparatively  short 
time  for  all   sewage   discharged   into   watercourses.     The   solid 

1  Moore  and  Silcock,  Sanitary  Engineering,  p.  67,  1909. 


6  INTRODUCTION 

matters  removed  by  treatment  may  be  buried,  burned,  dumped 
into  water,  or  used  as  a  fertilizer. 

If  the  volume  of  diluting  water,  or  the  area  and  character  of 
land  used  for  disposal  are  not  as  they  should  be,  a  nuisance  will 
be  created.  The  aim  of  all  methods  of  sewage  treatment  has  so 
far  been  to  produce  an  effluent  which  could  be  disposed  of  without 
nuisance  and  in  certain  exceptional  cases  to  protect  public  water 
supplies  from  pollution.  Financial  returns  have  been  sought 
only  as  a  secondary  consideration.  A  few  sewage  farms  and  irri- 
gation projects  might  be  considered  as  exceptions  to  this  as  the 
value  of  the  water  in  the  sewage  as  an  irrigant  has  been  the  primary 
incentive  to  the  promotion  of  the  farm. 

It  is  to  be  remembered  that  since  the  aim  of  all  sewage  treat- 
ment is  to  produce  an  effluent  that  can  be  disposed  of  without 
causing  a  nuisance,  the  simplest  process  by  which  this  result  can 
be  attained  under  the  conditions  presented  is  the  process  to  be 
adopted.  No  attempt  is  made  to  purify  sewage  completely,  or 
on  a  practical  scale  to  make  drinking  water. 

5.  Methods    of    Treatment. — Screening    and    sedimentation 
are  the  primary  methods  for  the  treatment  of  sewage.     By  these 
methods  a  portion  of  the  floating  and  settleable  solids  are  removed, 
preventing  the  formation  of  unsightly  scum  and  putrefying  sludge 
banks.     Chemicals  are  sometimes  added  to  the  sewage  to  form  a 
heavy  flocculent  precipitate  which  hastens  sedimentation  of  the 
solid  matters  in  the  sewage.     The  process  in  these  methods  is 
mechanical  and  the  solid  matters  removed  from  the  sewage  must 
be  disposed  of  by  other  methods  than  dilution  with  the  sewage 
effluent.     More  complete  methods  of  treatment  are  dependent  on 
biologic   action.     Under  these  methods   of  treatment   complete 
stabilization  of  the  effluent  is  approached,  and  in  the  most  com- 
plete treatment  an  effluent  is  produced  which  is  clear,  sparkling, 
non-odorous,  non-putrescible,  and  sterile.     Sterilization  of  sewage, 
usually  with  chlorine  or  some  of  its  compounds,  has  been  used,  not 
to  reduce  the  amount  of  diluting  water  necessary,  but  to  reduce 
the  number  of  pathogenic  germs  and  to  minimize  the  danger  of 
the  transmission  of  disease. 

6.  Definitions. — Sewage   and   sewerage   are  not   synonymous 
terms  although  frequently  confused.     Sewage  is  the  spent  water 
supply  of  a  community  containing  the  waste  from  domestic, 
industrial  or  commercial  use,  and  such  surface  and  ground  water 


DEFINITIONS  7 

as  may  enter  the  sewer.1  Sewerage  is  the  name  of  the  system  of 
conduits  and  appurtenances  designed  to  carry  off  the  sewage. 
It  is  also  used  to  indicate  anything  pertaining  to  sewers. 

A  difference  is  made  between  sanitary  sewage,  storm  sew- 
age, and  industrial  wastes.  Sanitary  sewage,  sometimes  called 
domestic  sewage,  is  the  liquid  wastes  discharged  from  residences 
or  institutions,  and  contains  water  closet,  laundry  and  kitchen 
wastes.  Storm  sewage  is  the  surface  run-off  which  reaches  the 
sewers  during  and  immediately  after  a  storm.  Industrial  wastes 
are  the  liquid  waste  products  discharged  from  industrial 
plants. 

A  sewer  is  a  conduit  used  for  conveying  sewage. 

The  names  of  the  conduits  through  which  sewage  may  flow 
are: 

Soil  Stack. — A  vertical  pipe  in  a  building  through  which  waste 
water  containing  fecal  matter  or  urine  is  allowed  to  flow. 

Waste  Pipe. — A  vertical  pipe  in  a  building  through  which 
waste  water  containing  no  fecal  matter  is  allowed  to  flow. 

House  Drain. — The  approximately  horizontal  portion  of  a 
house  drainage  system  which  conveys  the  drainage  from  the  soil 
stack  or  waste  pipe  to  the  point  of  discharge  from  the  build- 
ing. 

House  Sewer. — The  pipe  which  leads  from  the  outside  wall  of 
the  building  to  the  sewer  in  the  street. 

Lateral  Sewer. — The  smallest  branch  in  a  sewerage  system, 
exclusive  of  the  house  sewers. 

Sub-main  or  Branch  Sewer. — A  sewer  from  which  the  sewage 
from  two  or  more  laterals  is  discharged.2 

Main  or  Trunk  Sewer. — A  sewer  into  which  the  sewage  from 
two  or  more  sub-main  or  branch  sewers  is  discharged.3 

Intercepting  Sewer. — A  sewer  generally  laid  transversely  to  a 
sewerage  system  to  intercept  some  portion  or  all  of  the  sewage 
collected  by  the  system. 

Relief  Sewer. — A  sewer  intended  to  carry  a  portion  of  the  flow 
from  a  district  already  provided  with  sewers  of  insufficient  capacity 
and  thus  preventing  overtaxing  the  latter.4 

1  Similar  to  the  definition  proposed  by  the  Am.  Public  Health 

2  Definition  recommended  by  Am.  Public  Health  Assn. 

3  Ibid. 

4  Ibid. 


8  INTRODUCTION 

Outfall  Sewer. — That  portion  of  a  main  or  trunk  sewer  below 
all  branches. 

Flushing  Sewer. — A  conduit  through  which  water  is  conveyed 
for  flushing  portions  of  a  sewerage  system. 

Force  Main. — A  conduit  through  which  sewage  is  pumped 
under  pressure. 


CHAPTER  II 
WORK  PRELIMINARY  TO  DESIGN 

7.  Division  of  Work. — Engineering  work  on  sewerage  can  be 
divided  into  four  parts,  namely:  preliminary,  design,  construc- 
tion and  maintenance.     An  engineer  may  be  engaged  during 
any  one  or  all  of  these  periods  on  the  same  sewerage  system,  and 
should  therefore  be  acquainted  with  his  duties  during  each  period. 

8.  Preliminary. — The  demand  for  sewerage  normally  follows 
the  installation  or  extension  of  the  public  water  supply.     It  may 
be  caused  by:  a  lack  of  drainage  on  some  otherwise  desirable 
tract  of  real  estate;   from  a  public  realization  of  unpleasant  or 
unhealthful  conditions  in  a  built-up   district;    or  through  the 
realization  by  the  municipal  administration  of  the  necessity  for 
caring  for  the  future.     In  whatever  way  the  demand  may  be 
created  the  engineer  should  take  an  active  part  in  the  promotion 
of  the  work. 

The  engineer's  duties  during  the  preliminary  period  are:  to 
make  a  study  of  the  possible  methods  by  which  the  demand  for 
sewerage  can  be  satisfied ;  to  present  the  results  of  this  study  in 
the  form  of  a  report  to  the  committee  or  organization  responsible 
for  the  promotion  of  the  work;  and  so  to  familiarize  himself  with 
the  conditions  affecting  the  installation  of  the  proposed  plans 
as  to  be  able  to  answer  all  inquiries  concerning  them.  This  work 
will  require  the  general  qualities  of  character,  judgment,  efficiency 
and  the  understanding  of  men  in  addressing  interested  persons 
individually  and  collectively  on  the  features  of  the  proposed 
plans,  and  the  exercise  of  engineering  technique  in  the  survey 
and  the  drawing  of  the  plans.  The  engineer  should  assure  him- 
self that  all  legal  requirements  in  the  drawing  of  petitions,  adver- 
tising, permits,  etc.,  have  been  complied  with.  This  requires 
some  knowledge  of  national,  state,  and  local  laws.  Although 
none  the  less  essential  their  description  is  not  within  the  scope  of 
this  book. 

9 


10  WORK  PRELIMINARY  TO  DESIGN 

The  engineer's  preliminary  report  should  contain  a  section 
devoted  to  the  feasibility  of  one  or  more  plans  which  may  be 
explained  in  more  or  less  detail  with  a  statement  of  the  cost  and 
advantages  of  each.  A  conclusion  should  be  reached  as  to  the 
most  desirable  plan  and  a  recommendation  made  that  this  plan  be 
insta  led.  Other  sections  of  the  report  may  be  devoted  to  a  history 
of  the  growing  demand,  a  description  of  the  conditions  necessitat- 
ing sewerage,  possible  methods  of  financing,  and  such  other  sub- 
jects as  may  be  pertinent.  The  making  of  the  preliminary  plan 
and  the  design  of  sewerage  works  are  described  in  subsequent 
chapters. 

9.  Estimate  of  Cost. — In  making  an  estimate  of  cost  the 
information  should  be  presented  in  a  readable  and  easily  compre- 
hended manner.  It  is  necessary  that  the  items  be  clearly  defined 
and  that  all  items  be  included.  The  method  of  determining  the 
costs  of  doubtful  items  such  as  depreciation,  interest  charges, 
labor,  etc.,  and  the  probability  of  the  fluctuation  of  the  costs  of 
certain  items  should  be  explained. 

The  engineer's  estimate  may  be  divided  somewhat  as  follows: 

Labor. 

Material. 

Overhead.  This  may  include  construction  plant, 
office  expense,  supervision,  bond,  interest  on  borrowed 
capital,  insurance,  transportation,  etc.  The  amount  of 
the  item  is  seldom  less  than  15  per  cent  and  is  usually 
over  20  per  cent  of  the  contract  price. 

Contingencies.  This  allowance  is  usually  10  to  15  per 
cent  of  the  contract  price. 

Profit.  This  should  be  from  5  to  10  per  cent  of  the 
sum  of  the  four  preceding  items. 

The  contract  price  is  the  sum  of  these  items.  To  this  may  be 
added: 

Engineering.     2  to  5  per  cent  of  the  contract  price. 
Extra  Work.     Zero  to  15  per  cent  of  the  contract  price ; 
dependent  on  the  character  of  the  work,  the  completeness 
of  the  preliminary  information,  the  completeness  of  the 
plans,  etc. 

Legal  expense. 

Purchase  of  land,  rights  of  way,  etc.,  etc. 

The  cost  of  the  sewer  may  be  stated  as  so  much  per  linear 
foot  for  different  sizes  of  pipe,  including  all  appurtenances 


ESTIMATE  OF  COST  11 

such  as  manholes,  catch-basins,  etc.,  or  the  items  may  be  sep- 
arated in  great  detail  somewhat  as  follows : 

Earth  excavation,  per  cu.  yd. 

Rock  excavation,  per  cu.  yd. 

Backfill,  per  cu.  yd. 

Brick  manholes,  3  feet  by  4  feet,  per  foot  of  depth. 

Vitrified  sewer  pipe  with  cement  joints,  in  place, 

inches  in  diameter,  0  to  6    feet  deep 

6  to  8    feet  deep 
8  to  10  feet  deep 

Repaving,  macadam  per  sq.  yd. 
asphalt  per  sq.  yd. 

Flush  tanks, gal.  capacity,  per  tank. 

Service  pipes  to  flush  tanks,  per  linear  foot.,  etc.,  etc. 

These  methods  represent  the  two  extremes  of  presenting  cost 
estimates.  Each  method,  or  modification  thereof,  may  have  its 
use,  dependent  on  circumstances. 

Reliable  cost  data  are  difficult  to  obtain.  Lists  of  prices  of 
materials  and  labor  are  published  in  certain  engineering  and  trade 
periodicals.  The  Handbook  of  Cost  Data  by  H.  P.  Gillette 
contains  lists  of  the  amount  of  material  and  labor  used  on  certain 
specific  jobs  and  types  of  construction.  The  price  of  labor  and 
materials  on  the  local  market  can  be  obtained  from  the  local 
Chamber  of  Commerce,  contractors  and  other  employers  of  labor, 
and  dealers  in  the  desired  commodities.  Contract  prices  for 
sewerage  work  published  in  the  construction  news  sections  of 
engineering  periodicals  may  be  a  guide  to  the  judgment  of  the 
probable  cost  of  proposed  work,  but  are  generally  dangerous  to 
rely  upon  as  full  details  are  lacking  in  the  description  of  the  work. 
A  wide  experience  in  the  collection  and  use  of  cost  data  is  the 
desirable  qualification  for  making  estimates  of  cost.  It  is  pos- 
sessed by  few  and  is  not  an  infallible  aid  to  the  judgment. 

Having  completed  the  design  and  summary  of  the  bills  of 
material  and  labor  necessary  for  each  structure  or  portion  of  the 
sewerage  system,  the  product  of  the  unit  cost  and  the  amount 
of  each  item  plus  an  allowance  for  overhead  will  equal  the  cost 
of  the  item.  The  total  cost  will  be  the  sum  of  the  costs  of  each 
item.  The  items  should  be  so  grouped  that  the  cost  of  the  differ- 
ent portions  of  the  system  are  separated  in  order  that  the  effect 
on  the  total  cost  resulting  from  different  combinations  of  items 
or  the  omission  of  any  one  item  may  be  readily  computed. 


12  WORK  PRELIMINARY  TO  DESIGN 

A  method  for  estimating  the  approximate  cost  of  sewers, 
devised  by  W.  G.  Kirchoffer1  depends  upon  the  use  of  the  diagram 
shown  in  Fig.  2.  The  factors  for  local  conditions  are  shown  in 
Table  2.  For  example,  let  it  be  required  to  find  the  cost  of  a 
15-inch  vitrified  pipe  sewer  at  a  depth  of  9  feet,  if  the  unit  costs 

Diameter  of  Vitrified  Pipe  in  Inches. 
4      6     8      10     12     14     16     18    20    22    24    26    28     30    32    34    36 


30 


20  30  40  50  60  70  80  90  j^ 


2  3456 

'Cost  of  Sey/er  in  Dollars  per  Lineal  Foot. 


7    8    9  10  II 12  14  16  IS 


FIG.  2. — Diagram  for  Estimating  the  Cost  of  Sewers. 

Eng.  News,  Vol.  76,  p.  781. 

of  labor  and  material  and  the  conditions  are  the  same  as  shown 
in  Table  3. 

Solution 

First:  To  find  the  factor  depending  on  local  condi- 
tions, enter  the  diagram  at  the  10-inch  diameter  and 
continue  down  until  the  intersection  with  the  depth  of 
trench  at  8.2  feet  is  found.  Now  go  diagonally  parallel 
to  lines  running  from  left  to  right  upwards  to  the  inter- 

1  Eng.  News,  Vol.  76,  1916,  p.  781.    See  also  Eng.  News-Record,  Vol.  85, 
1920,  pp.  22,  1175. 


ESTIMATE  OF  COST 


13 


section  with  the  vertical  line  through  a  cost  of  45  cents 
per  foot.  The  diagonal  line  running  from  left  to  right 
downwards  through  this  intersection  corresponds  to  a 
factor  of  about  11. 


TABLE  <*, 

FACTORS  FOR  COSTS  OF  SEWERS  TO  BE  USED  WITH  FIGURE  2 


Character  of  Material 

Factor 

Character  of  Material 

Factor 

Clay,    gravel    and    boulders, 
Medford  
Mostly    sand,    deep   trenches 

22-26 

Clay  2  miles  inland.  Laborers 
boarded     at     sanitarium, 
Wales 

35 

sheeted.       Wages  medium. 
Richland  Center  

21-22 

Clay,  gravel  and  boulders  at 
Plymouth      

20-27 

Sandy  clay.     Wages  medium. 
Labor   conditions  good  at 
Kiel            

15-20 

Sand,  clay  and  good  digging 
at  Lake  Mills  
Red  clay     Machine  work  at 

16-19 

Sand.      Sandy  clay,    some 

North  Milwaukee  

20-24 

water.      Labor    conditions 
good.     Pipe  prices  medium 
at  Manston 

14-20 

Good  digging.      Wages  me- 
dium at  West  Salem  
Sandy  soil    bracing  only  re- 

17-19 

Gravelly    clay,    i^th  laid  in 
concrete  at  Burlington 

13-22 

quired.    No  water.  Wages 
and  pipe  medium 

14 

Sandy  clay,  some  water,  sheet- 

Red sticky  clay  

24 

ing  at  La  Farge  
Sand  with  water 

17-23 
20 

Good    digging    in    any   soil 
Work  scarce. 

15 

Gravel  and  boulders.     High 
wages  

26 

Red  clay.     No  bracing  
Work  inland  from  railroad 

20 

Clay  soil.     Good  digging.  .  .  . 
Sandy  clay.     Some  water.  .  .  . 

17 
23 

Boarding    laborers     and 
other  expenses  

35 

Second:  To  find  the  cost  of  15-inch  pipe  at  a  depth  of 
9.0  feet,  enter  the  diagram  at  a  diameter  of  15  inches 
and  continue  down  until  the  intersection  with  a  depth  of 
trench  at  9  feet  is  found.  Now  go  diagonally  parallel  to 
lines  running  from  left  to  right  upwards  to  the  intersection 
with  the  diagonal  line  running  from  left  to  right  downwards 
corresponding  to  the  factor  of  11  found  above.  The 
vertical  line  passing  through  this  point  shows  the  cost  to 
be  67  cents  per  foot. 


14 


WORK  PRELIMINARY  TO  DESIGN 


TABLE  3 

COST  OF  SEWER  CONSTRUCTION  AT  ATLANTIC,  IOWA 
(From  Gillette's  Handbook  of  Cost  Data) 

Material :  Clay,  not  difficult  to  spade  and  requiring  little  or  no  bracing  and 
practically  no  pumping.  All  hand  work  except  backfill  which  was  done  by 
team  and  scraper.  Depth  of  trench  averaged  8.2  feet;  width  30  inches. 
Diameter  of  pipe  10  inches. 


Wage, 

Cost, 

Wage, 

Cost, 

Item 

Cents 

Cents 

Item 

Cents 

Cents 

per 

per 

per 

per 

Hour 

Foot. 

Hour 

Foot. 

Pipe 

0.20 

Trenching.     Bracing 

Hauling    team    and 

men 

17 

002 

driver                •  • 

30 

003 

Backfilling     Shovel 

17 

010 

Hauling.    Man  help- 

Backfilling.      Team 

ing 

17 

001 

and  scraper 

30 

008 

Cement  and  sand  .  .  . 

.006 

Backfilling.         Man 

Pipe  layers  

22 

.014 

and  scraper.  .  .  . 

17 

005 

Pipe  layer's  helper.  . 

17 

.014 

Water  boy  

10 

.006 

Trenching.  Top  men 

17 

.027 

Foreman  

30 

022 

men 

17 

130 

Total 

450 

Trenching.    Scaffold 

17 

.002 

men    .  .        

17 

002 

METHODS  OF  FINANCING 

The  construction  of  sewerage  works  may  be  paid  for  by  the 
issue  of  municipal  bonds,  by  special  assessment,  by  funds  available 
from  the  general  taxes,  or  by  private  enterprise. 

10.  Bond  Issues. — A  municipal  bond  is  a  promise  by  the 
municipality  to  pay  the  face  value  of  the  bond  to  the  holder  at  a 
certain  specified  time,  with  interest  at  a  stipulated  rate  during 
the  interim.  The  security  on  the  bond  is  the  taxable  property 
in  the  municipality.  The  legal  restrictions  thrown  around  muni- 
cipal bond  issues,  the  value  of  the  taxable  property  in  the  munici- 
pality, all  of  which  may  be  used  as  security  for  municipal  bonds, 
and  the  fact  that  a  municipality  can  be  sued  in  case  of  default, 
make  municipal  bonds  desirable  and  provide  a  good  market  for 


SPECIAL  ASSESSMENT  15 

their  sale.  The  funds  available  from  a  municipal  bond  issue  are 
limited  by  the  amount  that  the  legal  limit  is  in  excess  of  the  out- 
standing issues.  The  legal  limit  varies  in  different  states  from 
about  5  to  15  per  cent  of  the  assessed  value  of  the  property  in 
the  municipality.  In  some  cases  the  amount  available  from 
municipal  bonds  has  been  increased  by  forming  a  municipality 
within  a  municipality  such  as  a  sanitary  district,  a  park  district, 
a  drainage  district,  etc.,  which  comprises  a  large  portion  or  all 
of  an  existing  municipal  corporation.  This  case  is  well  illustrated 
in  some  parts  of  the  City  of  Chicago  where  the  municipal  taxing 
powers  are  shared  by  the  City  government,  the  Sanitary  District, 
and  Park  Commissioners.  The  right  to  create  a  new  municipal 
corporation  must  be  granted  by  the  state  legislature.  Knowledge 
of  fixed  bonds,  serial  bonds,  life  of  bonds,  sinking  funds,  etc.  is  an 
important  part  of  an  engineer's  education.1 

Bond  issues  must  usually  be  presented  to  the  voters  for  approval 
at  an  election.  If  approved,  and  other  legal  procedure  has  been 
followed,  the  bonds  may  be  bought  by  some  of  the  many  bonding 
houses,  or  by  private  individuals,  and  the  money  is  immediately 
available  for  construction.  The  bonds  are  redeemed  by  general 
taxation  spread  over  the  period  of  the  issue. 

11.  Special  Assessment. — A  special  assessment  is  levied  against 
property  benefited  directly  by  the  structure  being  paid  for. 
Special  assessments  are  used  for  the  payment  for  the  construction 
of  lateral  sewers  which  are  a  direct  benefit  to  separate  districts 
but  are  without  general  benefit  to  the  city.  In  case  the  construc- 
tion of  an  outfall  sewer  or  the  erection  of  a  treatment  plant, 
which  may  be  of  some  general  benefit,  is  necessary  to  care  for  a 
separate  district,  a  part  of  the  expense  may  be  borne  by  funds 
available  from  general  taxation.  The  legal  procedure  for  the 
raising  of  funds  by  special  assessment  and  the  purpose  to  which 
the  funds  so  raised  may  be  applied  are  stipulated  in  great  detail 
in  different  states  and  their  directions  must  be  followed  implicitly. 
Illinois  procedure,  which  is  similar  to  that  in  some  other  states, 
is  as  follows :  a  meeting  of  the  interested  property  owners  is  called 
by  a  committee  or  board  of  the  municipal  government,  as  the 
result  of  a  petition  by  interested  persons  or  through  the  inde- 
pendent action  of  the  Board.  At  this  preliminary  meeting  or 

1  For  a  more  extensive  treatment  of  the  subject  see  Principles  and  Methods 
of  Municipal  Administration  by  W.  B.  Munro,  1916. 


16  WORK  PRELIMINARY  TO  DESIGN 

public  hearing  arguments  for  and  against  the  proposed  improve- 
ment are  heard.  The  engineer  is  present  at  this  meeting  to 
answer  questions  and  to  advise  concerning  the  engineering 
features  of  the  plan.  If  approval  is  given  by  the  Board  the  plan 
and  specifications  are  prepared  complete  in  every  detail  and 
incorporated  in  an  ordinance  which  is  presented  to  the  legislative 
branch  of  the  city  government  for  passage.  If  the  project  is 
adopted  it  is  taken  to  the  county  court.  An  assessment  roll  is 
prepared  by  a  commissioner  appointed  by  the  court.  This  roll 
shows  the  amount  to  be  assessed  against  each  piece  of  property 
benefited.  A  hearing  is  then  held  in  the  county  court  at  which 
the  owner  of  any  assessed  property  may  voice  objections  to  the 
.continuation  of  the  project.  The  project  may  be  thrown  out  of 
cougrt  for  many  different  reasons,  such  as  the  misspelling  of  a  street 
name,  an  error  in  an  elevation,  an  error  in  the  description  of  a 
pavement,  but  most  important  of  all  is  definite  proof  that  the 
benefit  is  not  equal  to  the  assessment.  The  many  minor  irregu- 
larities which  may  nullify  the  procedure  in  a  special  assessment 
differ  in  different  states  and  in  different  courts  in  the  same  state, 
but  in  general  no  court  can  approve  an  assessment  greater  than 
the  benefits  given.  After  the  project  has  passed  through  the 
county  court  and  the  assessment  roll  has  been  approved,  bonds 
may  be  issued  for  the  payment  of  the  contractor.  Special  assess- 
ment bonds  are  liens  against  the  property  assessed  and  have  not 
the  same  security  as  a  general  municipal  bond.  For  this  reason 
a  city  which  has  reached  its  legal  limit  of  municipal  bond  issues 
can  still  pay  for  work  by  special  assessment. 

The  funds  available  from  special  assessments  are  limited  only 
by  the  benefit  to  the  property  assessed.  The  amount  of  the 
benefit  is  difficult  to  fix  and  may  lead  to  much  controversy.  It 
should  not  exceed  the  amount  demanded  for  similar  work  in  other 
localities,  unless  unusual  and  well-understood  reasons  can  be 
given. 

12.  General  Taxation. — In  paying  for  public  improvements 
by  general  taxation  the  money  is  taken  from  the  general  municipal 
funds  which  have  been  apportioned  for  that  purpose  by  the 
legislative  department  of  the  municipal  government.  This 
method  of  raising  funds  for  sewerage  construction  is  seldom  used 
unless  the  political  situation  is  unfavorable  to  the  success  of  a 
bond  issue  or  special  assessment  and  the  need  for  the  improvement 


PREPARING  FOR  DESIGN  17 

is  great.  It  is  usually  difficult  to  appropriate  sufficient  funds  for 
new  construction  as  the  general  tax  is  apportioned  to  support 
only  the  operating  expenses  of  the  city,  and  statutory  provisions 
limit  the  amount  of  tax  which  can  be  levied. 

13.  Private  Capital. — Private  capital  has  been  used  for  financ- 
ing sewerage  works  in  some  cases  because  of  the  aversion  of  the 
public  in  some  cities  to  the  payment  of  a  tax  for  the  negative 
service  performed  by  a  sewer.     Sewers  are  buried,  unseen,  and 
frequently  forgotten,  but  knowledge  of  their  necessity  has  spread 
and  the  number  of  privately  owned  sewerage  works  is  diminishing 
because  of  the  better  service  which  can  be  provided  by  the  munici- 
pality. 

Franchises  are  granted  to  private  companies  for  the  construc- 
tion of  sewers  only  after  the  city  has  exhausted  other  methods  for 
the  raising  of  capital.  The  return  on  the  private  capital  invested 
is  received  from  a  rental  paid  by  the  city,  or  paid  directly  by  the 
users  of  the  system,  an  initial  payment  usually  being  demanded 
for  connection  to  the  system.  To  be  successful  the  enterprise 
must  be  popular  and  must  fill  a  great  need.  This  method  of 
financing  sewerage  works  is  seldom  employed  as  favorable  con- 
ditions are  not  common. 

PRELIMINARY  WORK 

14.  Preparing  for  Design. — Methods  for  the  design  of  sewerage 
systems  are  given  in  Chapter  V.     Before  the  design  is  made 
certain  information  is  essential.     A  survey  must  be  made  from 
which  the  preliminary  map  can  be  prepared  as  described  in  Art. 
42.     Other  necessary  information  which  is  the  basis  of  subsequent 
estimates  of  the  quantity  of  sewage  to  be  cared  for  must  be  obtained 
by  a  study  of  rates  of  water  consumption  and  the  density  and 
growth  of  population,  the  measurement  of  the  discharge  from 
existing  sewers,  and  the  compilation  of  rainfall  and  run-off  data. 
If  no  rainfall  data  are  available  estimates  must  be  made  from 
the  nearest  available  data.     Observations  of  rainfall  or  run-off 
for  periods  of  less  than  10  to  20  years  are  likely  to  be  misleading. 
Methods  for  gathering  and  using  this  information  are  explained 
in  subsequent  chapters. 

Underground   surveys   are   desirable   along  the  lines  of  the 
proposed   sewers  to   learn   of  obstructions,    difficult  excavation 


18  WORK  PRELIMINARY  TO  DESIGN 

and  other  conditions  which  may  be  met.  All  such  data  are  seldom 
gathered  except  for  sewerage  systems  involving  the  expenditure 
of  a  large  amount  of  money.  For  construction  in  small  towns 
or  small  extensions  to  an  existing  system  the  funds  are  usually 
insufficient  for  extensive  preliminary  investigation.  The  saving 
in  this  respect  is  paid  unknowingly  to  the  contractor  as  com- 
pensation for  the  risk  in  bidding  without  complete  information. 

15.  Underground  Surveys. — These  may  be  more  or  less  exten- 
sive dependent  on  the  character  of  the  district  in  which  construc- 
tion is  to  take  place.  In  built-up  districts  the  survey  should  be 
more  thorough  than  in  sparsely  settled  districts  where  only  the 
character  of  the  excavated  material  is  of  interest  and  no  obstruc- 
tions are  to  be  met. 

Underground  surveys  furnish  to  the  engineer  and  to  prospect- 
ive bidders  on  contract  work  information  on  which  the  design 
and  estimate  of  cost  and  the  contractor's  bid  may  be  based  and 
without  which  no  intelligent  work  can  be  done.  By  removing 
much  of  the  uncertainty  of  the  conditions  to  be  met  in  the  con- 
struction of  the  sewer,  the  design  can  be  made  more  economical 
and  the  contractor's  btd  should  be  markedly  lower,  sufficiently 
so  to  repay  more  than  the  expense  of  the  survey.  The  information 
to  be  obtained  consists  of  the  location  of  the  ground-water  level, 
and  the  location  and  sizes  of  water,  gas,  and  sewer  pipes,  tele- 
phone and  electric  conduits,  street-car  tracks,  steam  pipes,  and  all 
other  structures  which  may  in  any  way  interfere  with  subsurface 
construction.  These  structures  should  be  located  by  reference 
to  some  permanent  point  on  the  surface.  The  elevation  of  the 
top  of  the  pipes,  except  sewers,  rather  than  the  depth  of  cover 
should  be  recorded,  as  the  depth  of  cover  is  subject  to  change. 
The  elevation  of  sewers  should  be  given  to  the  invert  rather  than 
to  the  top  of  the  pipe. 

A  portion  of  the  map  of  the  subsurface  conditions  at  Wash- 
ington, D.  C.,  is  shown  in  Fig.  3.  Many  of  the  dimensions  and 
notations  are  not  shown  to  avoid  confusion  on  this  small  repro- 
duction.1 Colors  are  generally  used  instead  of  different  forms  of 
cross  hatching  to  show  the  different  classes  of  pipe  and  structures. 
In  addition  to  a  record  of  the  underground  structures  the  char- 
acter of  the  ground  and  the  pavement  should  be  recorded.  A 
comprehensive  underground  survey  is  seldom  available  nor  does 
1  Eng.  Record,  Vol.  74, 1916,  p.  263. 


UNDERGROUND  SURVEYS 


19 


ill 

!« 

0  S  5 


11, 


&  co         Q. 

3     co          <*,    sJ 

r*u 

S  s  8 1 H 


•a 


ij 


Is" 

If! 

;!*»• 
llt 


20 


WORK  PRELIMINARY  TO  DESIGN 


time  usually  permit  its  being  made  preliminary  to  the  design  of  a 
sewerage  system.  The  character  of  the  material  through  which 
the  sewer  is  to  pass  should  be  determined  in  all  cases. 

Underground  pipes  and  structures  are  located  by  excavations, 
which  may  be  quite  extensive  in  some  cases.  Their  position  is 
fixed  by  measurements  referred  to  manholes  and  other  under- 
ground  structures  which  are  somewhat  permanent 
in  position.  A  city  engineer  should  grasp  every 
opportunity  to  record  underground  structures  when 
excavations  are  made  in  the  streets.  The  character 
of  the  material  through  which  the  sewer  is  to  pass  is 
determined  by  borings. 

16.  Borings.  —  Methods  used  for  the  investigation 
of  subsurface  conditions  preliminary  to  sewer  con- 
struction are:  punch  drilling,  boring  with  earth 
auger,  jet  boring,  wash  boring,  percussion  drilling, 
abrasive  drilling,  and  hydraulic  drilling.  The  last 
three  methods  named  are  used  only  for  unusually 
deep  borings  or  in  rock. 

Punch  drills  are  of  two  sorts.  The  simplest  punch 
drill  consists  of  an  iron  rod  f  of  an  inch  to  1  inch  in 
diameter,  in  sections  about  4  feet  long.  One  section 
is  sharpened  at  one  end  and  threaded  at  the  other 
so  that  the  next  section  can  be  screwed  into  it  with- 
out  increasing  the  diameter  of  the  rod,  as  shown  in 
Fig.  4.  The  drill  is  driven  by  a  sledge  striking  upon 
a  piece  of  wood  held  at  the  top  of  the  drill  to  pre- 
vent injury  to  the  threads.  The  drill  should  be  turned  as  it  is 
driven  to  prevent  sticking.  It  is  pulled  out  by  a  hook  and  lever  as 
shown  in  Fig.  5.  It  is  useful  in  soft  ground  for  soundings  up  to 
8  to  12  feet  in  depth.  Another  form  of  punch  drill  described 
by  A.  C.  Veatch1  consists  of  a  cylinder  of  steel  or  iron,  one 
to  two  feet  long  split  along  one  side  and  slightly  spread.  The 
lower  portion  is  very  slightly  expanded  and  tempered  into  a 
cutting  edge.  In  use  it  is  attached  to  a  rope  or  wooden  poles 
and  lifted  and  dropped  in  the  hole  by  means  of  a  rope  given  a  few 
turns  about  a  windlass  or  drum.  By  this  process  the  material 
is  forced  up  into  the  bit,  slightly  springs  it,  and  so  is  held.  When 
the  bit  is  filled  it  is  raised  to  the  surface  and  emptied.  Much 
1  Professional  paper  No.  46,  United  States  Geological  Survey,  1906,  p.  97. 


FIG.  4. 
Punch  Drill. 


BORINGS 


21 


FIG.  5, — Lever  for  Pulling  Punch  Drill. 


deeper  holes  can  be 

made  with  this  than 

with  the  sharpened 

solid  rod. 

Types  of  earth 

augers     about    1J- 

inches  in  diameter 

are  shown  in  Fig.  6. 

They    are    screwed 

on  to   the    end    of 

a    section    of    the 

pipe  or  rod  and  as  the  hole  is  deepened  successive  lengths  of  pipe 

or  rod  are  added.     The  device  is  operated  by  two  men.     It  is 

pulled  by  straight  lifting  or  with  the  assistance  of  a  link  and 

^^     '_  j.      ^      ^_.,CT lever  similar  to  that  shown 

I  in  Fig.  5.     The  device  is 

suitable  for  soft  earth  or 
sand  free  from  stones,  and 
can  be  used  for  holes  T5 
to  25  feet  in  depth.  For 
deeper  holes  a  block  and 
tackle  should  be  used  for 
lifting  the  auger  from  the 
hole.  It  is  not  suitable 
for  holes  deeper  than  about 
35  feet. 

In  the  jetting  method 
water  is  led  into  the  hole 
through  a  f -inch  or  1-inch 
pipe,  and  forced  down- 
ward through  the  drill  bit 
or  nozzle  against  the  bot- 
tom of  the  hole.  The 
complete  equipment  is 
shown  in  Fig.  7.1  It  is 
not  always  necessary  to 
case  the  hole  as  shown  in 
the  figure  as  the  muddy 

FIG.  6. — Earth  Augers.  water  and    the    vibration 

1  United  States  Geological  Survey,  Water  Supply  paper  No.  257,  1911. 


22 


WORK  PRELIMINARY  TO  DESIGN 


Drive  Weight 
Wooden  Platform 
Clamped  to 
Casing 


Wooden  Buffer 
Drive  Head 
Force  Pump 


-2- 

Z)r/Ve  Weight  Rope 

to  Hoisting  Drum  orSpool 

-Drill  Rod  Pope 
to  Hoisting  Drum 


-  — Pope  Supporting  Drill  Rods 


Clamp orWrench  //       Qg.         ,'PubberHosefofa 


for  Turning 
Drill  Pipe  " 


Drive 
Weight—. 


Pump 


/  Pipe  Wrench  for 
<        Turning  Casing 

Poo  I  to  Supply  Pump 


FIG.  7. — Jetting  Outfit. 

U.  S.  Geological  Survey,  Water  Supply  Paper,  No.  257 

1.  Simple  Jetting  Outfit.  2.  Jetting  Process.  3.  Common  Jetting  Drill. 

4a  and  46.  Expansion  Bit  or  Paddy.  5.  Drive  Shoe. 


BORINGS  23 

of  the  pipe  puddle  the  sides  so  that  they  will  stand  alone.  The 
jet  pipe  may  be  churned  in  the  hole  by  a  rope  passing  over  a  block 
and  a  revolving  drum.  In  suitable  soft  materials  such  as  clay, 
sand,  or  gravel,  holes  can  be  bored  to  a  depth  of  100  feet  and 
samples  collected  of  the  material  removed.  An  objection  to  the 
method  is  the  difficulty  of  obtaining  sufficient  water. 

Methods  of  drilling  in  rock  up  to  depths  of  20  feet  are  described 
in  Chapter  XI  under  Rock  Drilling.  For  deeper  holes  percussion, 
abrasive,  or  hydraulic  methods  as  used  for  deep  well  drilling  must 
be  employed. 


CHAPTER  III 
QUANTITY  OF  SEWAGE 

17.  Dry-weather  Flow. — Estimates  of  the  quantity  of  dry- 
weather  sewage  flow  to  be  expected  are  ordinarily  based  on  the 
population,  the  character  of  the  district,  the  rate  of  water  con- 
sumption, and  the  probable  ground-water  flow.     Future  condi- 
tions are  estimated  and  provided  for,  as  the  sewers  should  have 
sufficient  capacity  to  care  for  the  sewage  delivered  to  them  during 
their  period  of  usefulness. 

18.  Methods  for  Predicting  Population. — Methods  for  the 
prediction  of  future  population  are  given  in  the  following  para- 
graphs. 

The  method  of  graphical  extension.  This  is  the  quickest  and 
most  simple  of  all.  In  this  method  a  curve  is  plotted  on  rect- 
angular coordinates  to  any  convenient  scale,  with  population  as 
ordinates  and  years  as  abscissas.  The  curve  is  extended  into 
the  future  by  judgment  of  its  general  tendency.  An  example  is 
given  of  the  determination  of  the  population  of  Urbana,  Illinois, 
in  1950.  Table  4  contains  the  population  statistics  which  have 
been  plotted  on  line  A  in  Fig.  8  and  extended  to  1950.  The 
probable  population  in  1Q50  is  shown  by  this  line  to  be  about 
21,000. 

The  method  of  geometrical  progression.  In  this  method  the 
rate  of  increase  during  the  past  few  years  or  decades  is  assumed 
to  be  constant  and  this  rate  is  applied  to  the  present  population 
to  forecast  the  population  in  the  future.  For  example  the  rate 
of  increase  of  population  in  UrbaTia  for  the  past  7  decades  has 
varied  widely,  but  indications  are  that  for  the  next  few  decades 
it  will  be  about  20  per  cent.  Applying  this  rate  from  1920  to 
1950  the  population  in  1950  is  shown  to  be  about  17,800.  It  is 
evident  that  this  method  may  lead  to  serious  error  as  insufficient 
information  is  given  in  the  table  to  make  possible  the  selection  of 
the  proper  rate  of  increase. 

24 


METHODS  FOR  PREDICTING  POPULATION 


25 


TABLE  4 

POPULATION  STUDIES 


Urbana,  Illinois 

Population  of 

Abso- 

Per 

Year 

Ann 

Popu- 
lation 

lute 
Increase 
for  Each 

Cent 
Increase 
forEach 

Decatur 

Dan- 
ville 

Cham- 
paign 

Kanka- 
kee 

Peoria 

Bloom- 
ington 

Arbor, 
Michi- 

Decade 

Decade 

gan 

1850 

210 

736 

5,095 

1,594 

1860 

2,038 

1828 

85.6 

3,839 

1,632 

1,727 

2,984 

14,045 

7,075 

5,097 

1870 

2,277 

239 

10.5 

7,161 

4,751 

4,625 

5,189 

22,849 

14,590 

7,368 

1880 

2,942 

665 

22.6 

9,547 

7,733 

5,103 

5,651 

29,259 

17,180 

8,061 

1890 

3,511 

569. 

16.2 

16,841 

11,491 

5,839 

9,025 

41,024 

20,484 

9,431 

1900 

5,728 

2217 

38.7 

20,754 

16,354 

9,098 

13,595 

56,100 

23,286 

14,509 

1910 

8,245 

2517 

30.5 

31,140 

27,871 

12,421 

13,986 

66,950 

25,786 

14,817 

1920 

10,230 

1985 

19.4 

43,818 

33,750 

15,873 

16,721 

76,121 

28,638 

19,516 

Decades  in  Life  of  City  of  Urbana 
1870    1880    1890     1900    1910    1920    1930    1940    1950 

50,000 
40,000 

<n 

V 

'•£• 

0 

30,000- 

to 

% 

C 

20,000  J 

« 

1 

10,000 

1° 

Peon 

a-*l 

<0 

c 
jj  20.000 

<4- 

0 

| 

i  10,000 
<f 

( 

s 

ir 

•Deca 

fur 

$>/ 

/ 

L 

anvill 

; 

mingh 

..'--' 

,1*1 

•A 

1 

u 

'/ 

Bloo 
rbana 

jn^,— 

•OguT 

rfjjfe 

*2 

•tfOt 

^x* 

{R 

f 

..   — 

f 



/I 

(  / 

'//^ 

-Ann  Arbor 

Chan 

paigj 

U*j£ 

iA* 

^ 

& 

-Kan 

<akee 

? 

'&:& 

,.-•* 

7Z 

*! 

«—•- 

^.» 

^'' 

X 

X 

0        0      40      30      20       10       0       10       20      30      40       50      b 
Years  Before  and  After  which  each  City  had  a  Population  of  10,230 

FIG.  8. — Diagram  Showing  Methods  for  Estimating  Future  Population. 


26  QUANTITY  OF  SEWAGE 

The  method  of  utilizing  a  decreasing  rate  of  increase.  This 
method  attempts  to  correct  the  error  in  the  assumption  of  a  con- 
stant rate  of  increase.  After  a  certain  period  of  growth,  as  the 
age  of  a  city  increases  its  rate  of  increase  diminishes.  In  applying 
this  knowledge  to  a  prediction  of  the  future  population  of  a  city 
the  population  curve  is  plotted,  as  in  the  graphical  method  and  a 
straight  line  representing  a  constant  rate  or  increase  is  drawn 
tangent  to  the  curve  at  its  end.  The  curve  is  then  extended  at  a 
flatter  rate  in  accordance  with  the  rate  of  change  of  a  similar 
nearby  larger  city.  This  method  has  not  been  applied  to  any  of 
the  cities  included  in  Table  4,  as  none  has  reached  that  limiting 
period  where  the  rate  of  increase  has  begun  to  diminish. 

The  method  of  utilizing  an  arithmetical  rate  of  increase.  This 
method  allows  for  the  error  of  the  geometrical  progression  which 
tends  to  give  too  large  results  for  old  and  slow-growing  cities. 
This  method  generally  gives  results  that  are  too  low.  The  abso- 
lute increase  in  the  population  during  the  past  decade  or  other 
period  is  assumed  to  continue  throughout  the  period  of  prediction. 
Applying  this  method  to  the  same  case,  the  increase  in  the  popula- 
tion during  the  past  decade  was  2,000.  Adding  three  times  this 
amount  to  the  population  in  1920,  the  population  of  Urbana  in 
1950  will  be  about  16,000. 

The  method  involving  the  graphical  comparison  with  other 
cities  with  similar  characteristics.  In  this  method  population 
curves  of  a  number  of  cities  larger  than  Urbana  but  having 
similar  characteristics,  are  plotted  with  years  as  abscissas  and 
population  as  ordinates,  with  the  present  population  of  Urbana 
as  the  origin  of  coordinates.  The  population  curve  for  Urbana 
is  first  plotted.  It  will  lie  entirely  in  the  third  quadrant  as  shown 
by  the  heavy  full  line  in  Fig.  8.  The  population  curves  of  some 
larger  cities  are  then  plotted  in  such  a  manner  that  each  curve 
passes  through  the  origin  at  the  time  their  population  was  the 
same  as  that  of  the  present  population  of  Urbana.  These  curves 
lie  in  the  first  and  third  quadrants.  The  population  curve  of  the 
city  in  question  is  then  extended  to  conform  with  the  curves  of 
older  cities  in  the  most  probable  manner  as  dictated  by  judgment. 
Such  a  series  of  plots  has  been  made  in  Fig.  8.  The  results  indi- 
cate that  the  population  of  Urbana  in  1950  will  be  about  25,500. 

The  last  method  described  will  give  the  most  probable  result 
as  it  is  the  most  rational.  For  quick  approximations  the  geo- 


EXTENT  OF  PREDICTION  27 

metrical  progression  is  used.     The   arithmetical  progression   is 
useful  only  as  an  approximate  estimate  for  old  cities.  , 

19.  Extent  of  Prediction. — The  period  for  which  a  sewerage 
system  should  be  designed  is  such  that  each  generation  bears  its 
share  of  the  cost  of  the  system.     It  is  unfair  to  the  present  genera- 
tion to  build  and  pay  for  an  extensive  system  that  will  not  be 
utilized  for  25  years.     It  is  likewise  unfair  to  the  next  generation  to 
construct  a  system  sufficient  to  comply  with  present  needs  only, 
and  to  postpone  the  payment  for  it  by  a  long  term  bond  issue. 
An  ideal  solution  would  be  to  plan  a  system  which  would  satisfy 
present  and  future  needs  and  to  construct  only  those  portions 
which  would  be  useful  during  the  period   of  the  bond   issue. 
Unfortunately  this  solution  is  not  practical,  because,  1st,  it  is  less 
expensive  to  construct  portions  of  the  system  such  as  the  outfall, 
the  treatment  plant,  etc.,  to  care  for  conditions  in  advance  of 
present  needs,  and  2nd,  the  life  of  practically  all  portions  of  a 
sewerage  system  is  greater  than  the  legal  or  customary  time  limit 
on  bond  issues. 

A  compromise  between  the  practical  and  the  ideal  is  reached 
by  the  design  of  a  complete  system  to  fulfill  all  probable  demands, 
and  the  construction  of  such  portions  as  are  needed  now  in  accord- 
ance with  this  plan.  The  payment  should  be  made  by  bond 
issues  with  as  long  life  as  is  financially  or  legally  practical,  but 
which  should  not  exceed  the  life  of  the  improvement. 

The  prediction  of  the  population  should  therefore  be  made 
such  that  a  comprehensive  system  can  be  designed  with  intelli- 
gence. Practice  has  seldom  called  for  predictions  more  than  50 
years  in  the  future. 

20.  Sources    of    Information    on    Population. — The    United 
States  decennial  census  furnishes  the  most  complete  information 
on  population.     Unfortunately  it  becomes  somewhat  old  towards 
the  end  of  a  decade.     More  recent  information  can  be  obtained 
from  local  sources.     Practically  every  community  takes  an  annual 
school  census  the  accuracy  of  which  is  fairly  reliable.     The  gen- 
eral tendencies  of  the  population  to  change  can  be  learned  by  a 
study  of  the  post  office  records  showing  the  amount  of  mail  matter 
handled  at  various  periods.     Local  chambers  of  commerce  and 
newspapers  attempt  to  keep  records  of  population,  but  they  are 
often  inaccurate.     Another  source  of  information  is  the  gross 
receipts  of  public  service  companies,  such  as  street  railways,  water, , 


28 


QUANTITY  OF  SEWAGE 


gas,  electricity,  telephone,  etc.  The  population  can  be  assumed 
to  .have  increased  almost  directly  as  their  receipts,  with  proper 
allowance  for  change  in  rates,  character  of  management,  and  other 
factors. 

21.  Density  of  Population.— So  far  the  study  of  population 
has  been  confined  to  the  entire  city.  It  is  frequently  necessary 
to  predict  the  population  of  a  district  or  small  section  of  a  city. 
A  direct  census  may  be  taken,  or  more  frequently  its  population 
is  determined  by  estimating  its  density  based  on  a  comparison 
with  similar  districts  of  known  density,  and  multiplying  this 


o     o 


FIG.  9. — Density,  Area,  and  Population,  Cincinnati,  Ohio.     1850  to  1950. 

density  by  the  area  of  the  district.  In  determining  the  density, 
statistics  of  the  population  of  the  entire  city  will  be  helpful  but 
are  insufficient  for  such  a  problem.  A  special  census  of  the  area 
involved  would  be  conclusive  but  is  generally  considered  too  expen- 
sive. A  count  of  the  number  of  buildings  in  the  district  can  be 
made  quickly,  and  the  density  determined  by  approximating  the 
number  of  persons  per  building.  Statistics  of  the  population  of 
various  districts  together  with  a  description  of  the  character  of 
the  district  are  given  in  Table  5. 

The  density  of  population  in  Cincinnati  from  1850  to  1913  with 
predictions  to  1950  is  given  in  Fig.  9.1     This  shows  the  densities 
for  the  entire  city  and  is  illustrative  of  the  manner  in  which  future 
1  From  Eng.  Cont.,  Vol.  41,  1914,  p.  698. 


DENSITY  OF  POPULATION 


29 


TABLE  5 
DENSITIES  OF  POPULATION 


City 


Character  of  District 


Area, 
Acres 


Density 
per  Acre 


Philadelphia  Thomas  Run.  Residential.  Most'y  pairs 
of  two  and  three-story  houses.  1204  acres 
settled 1,840  59 

Pine  Street.  Residential.  Mostly  solid 
four  to  six-story  houses.  156  acres 
settled 160  97 

Shunk  Street.  Residential.  Mostly  pairs 
of  two  and  three-story  houses.  539  acres 
settled 539  119 

Lombard  Street.  Tenements  and  hotels, 
145  acres  settled 147  113 

York  Street.  Residential  and  manufactur- 
ing. 354  acres  settled 358  94 

New  York  City  Residential.  Three-story  dwellings  with 
18-foot  frontage,  and  four-story  flats  with 

20-foot  frontage 100 

Residential.     Five-story  flats 520-670 

Residential.     Six-story  flats 800-1000 

Residential.  Six-story  apartments.  High 
class 300 

Chicago  1st  Ward.     Retail  and  commercial.     The 

"Loop" 1,440        20.5 

2d  Ward.  Commercial  and  low-class  resi- 
dential solidly  built  up 800  53 . 5 

3d  Ward.     Low-class  residential 960        48. 1 

5th   Ward.      Industrial.      Some   low-class 

residences.     Not  solidly  built  up 2,240        25 . 51 

6th  Ward.  Residential.  Four  and  five- 
story  apartments.  A  few  detached  resi- 
dences    1,600  47 . 0 

7th  Ward.    Same  as  Ward  6.    Not  solidly 

built  up.     Contains  a  large  park 4,160        21.7 

8th  Ward.    Industrial.    Sparsely  settled ..    13,624          4.8 
9th  Ward.    Industrial  and  low-class  resi- 
dential.    Solidly  built  up 640        70.0 

10th  Ward.     Same  as  Ward  9 640        80.8 

13th  Ward.  Low-class  residential.  Solidly 
built  with  three  and  four-story  flats 6.100  36 . 7 


30 


QUANTITY  OF  SEWAGE 


TABLE  5 — Continued 
DENSITIES  OF  POPULATION 


Citv 


Character  of  District 


Area, 
Acres 


Density 
per  Acre 


Chicago  16th     Ward.       Middle-class     residential. 

Some  industries.     Well  built  up 800        81.5 

19th  Ward.     Industrial  and  commercial. 

Some-low  class  residences 640        90 . 7 

20th  Ward.    Low-class  residential.    Some 

industries.     Entirely  built  up 800        77 . 1 

21st  Ward.  Industrial.  Entirely  built  up  960  49.9 
23d  Ward.  Industrial  and  residential.  .  .  .  800  55.4 
24th  Ward.  Residential  apartment  houses 

and  middle-class  residences 1,120        46. 8 

25th     Ward.       Residential.       High-class 
apartments.    Wealthy  homes.    Contains 

a  large  park 4,160        24.0 

26th    Ward.      Residential.      Middle-class 
homes  and  apartments.    Fairly  well  built 

up 4,640        16.1 

27th  Ward.    Residential.    Sparsely  settled .   20,480          5.5 
29th  Ward.     Low-class  residential.     Two- 
story  frame  houses.    "Back  of  the  Yards"     6,400        12.8 

30th  Ward.     The  Stock  Yards 1,280        40. 1 

32d  Ward.    Scattered  residences 8,480          8 . 3 

33d  Ward.    Scattered  residences 12,944          5 . 5 

35th  Ward.    Scattered  residences 4,960        12.0 

General  average   The  most  crowded  conditions  with  five- 
story  and  higher,  contiguous  buildings  in 

poor  class  districts 750-1000 

Five  and  six-story  contiguous  flat  buildings 500-  750 

Six-story  high-class  apartments 300-  500 

Three  and  four-story  dwellings,  business 
blocks    and    industrial    establishments. 

Closely  built  up 100-  300 

Separate  residences,  50  to  75-foot  fronts, 
commercial    districts,    moderately    well 

built  up 50-  100 

Sparsely    settled    districts    and    scattered 
frame  dwellings  for  individual  families j     0-     50 


CHANGES   IN   AREA  31 


conditions  were  predicted  for  the  design  of  an  intercepting  sewer. 
The  data  given  in  Table  5  are  of  value  in  estimating  the  densities 
of  population  in  various  districts.  The  Committee  on  City  Plan 
of  the  Board  of  Estimate  and  Apportionment  of  New  York  City 
obtained  some  valuable  information  on  this  point,  especially  in 
Manhattan.  Three-story  dwellings  with  18-foot  frontage,  or  four- 
story  flats  with  20-foot  frontage,  presumably  contiguous,  were 
found  to  hold  100  persons  to  the  acre.  Five-story  flats  held  520 
to  670  persons  per  acre.  Six-story  flats  held  800  to  1,000  persons 
per  acre,  and  high  class  six-story  apartments  held  less  than  300 
per  acre. 

22.  Changes  in  Area. — In  order  to  determine  the  probable 
extent  of  a  proposed  sewerage  system  it  is  important  to  estimate 
the  changes  in  the  area  of  a  city  as  well  as  the  changes  in  the 
population.     With  the  same  population  and  an  increased  area 
the  quantity  of  sewage  will  be  increased  because  of  the  larger 
amount  of  ground  water  which  will  enter  the  sewers.     Predictions 
of  the  area  of  a  city  are  less  accurate  than  predictions  of  popula- 
tion because  the  factors  affecting  changes  cannot  be  so  easily 
predicted.     An  area  curve  plotted  against  time  would  be  helpful 
in  guiding  the  judgment,  but  its  extension  into  the  future  based 
on  past  occurrences  would  be  futile.     A  knowledge  of  the  city, 
its  political  tendencies,  possibilities  of  extension,  and  other  factors 
must  be  weighed  and  judged.     The  engineer,  if  he  is  ignorant  of 
the  city  for  which  he  is  making  provision,  is  dependent  upon  the 
testimony  of  real  estate  men,  business  men  and  others  acquainted 
with  the  local  situation. 

23.  Relation   between   Population   and   Sewage   Flow. — The 
amount  of  sewage  discharged  into  a  sewerage  system  is  generally 
equal  to  the  amount  of  water  supplied  to  a  community,  exclusive 
of  ground  water.     The  entire  public  water  supply  does  not  reach 
the  sewers,  but  the  losses  due  to  leakage,  lawn  sprinkling,  manu- 
facturing processes,  etc.,  are  made  up  by  additions  from  private 
water  supplies,   surface  drainage,  etc.     The  estimated  quantity 
of  water  used  but  which  did  not  reach  the  sewers  in  Cincinnati 
is  shown  in  Table  6.     The  amount  shown  represents  38  per  cent 
of  the  total  consumption.     Unless  direct  observations  have  been 
made  on  existing  sewers  or  other  factors  are  known  which  will 
affect  the  relation  between  water  supply  and  sewage,  the  average 
sewage  flow  exclusive  of  ground  water,  should  be  taken  as  the 


32 


QUANTITY  OF  SEWAGE 


average  rate  of  water  consumption.     Experience  has  shown  that 
water  consumption  increases  after  the  installation  of  sewers. 

TABLE  6 

ESTIMATED  QUANTITY   OF  WATER  USED  BUT  NOT  DISCHARGED  INTO  THE 
SEWERS  IN  CINCINNATI 

Expressed  in  gallons  per  capita  per  day,  and  based  on  a  total  consumption 
of  125  to  150  gallons  per  capita  per  day. 


Steam  railroads 

6  to    7 

Manufacturing     and     me- 

Street sprinklers  
Consumers  not  sewered  .  .  . 

6  to    7 
9  to  10£ 

chanical  
Lawn  sprinklers 

6  to  7 
3  to  3^ 

Leakage 

18  to  21 

The  public  water  supply  is  generally  installed  before  the  sewer- 
age system.  By  collecting  statistics  on  the  rate  of  supply  of 
water  a  fair  prediction  can  be  made  of  the  quantity  of  sewage 
which  must  be  cared  for.  The  rate  of  water  supply  varies  widely 
in  different  cities.  It  is  controlled  by  many  factors  such  as  meters, 
cost  and  availability  of  water,  quality  of  water,  climate,  popula- 
tion, etc.  In  American  cities  a  rough  average  of  consumption  is 
100  gallons  per  capita  per  day.  Other  factors  being  equal  the 
rate  of  consumption  after  meters  have  been  installed  will  be 
about  one-half  the  rate  before  the  meters  were  installed.  Low 
cost,  good  quantity  and  good  quality  will  increase  the  rate  of 
consumption,  and  the  rate  will  increase  slowly  with  increasing 
population.  Statistics  of  rates  of  water  consumption  are  given 
in  Table  7. 

24.  Character  of  District. — The  various  sections  of  a  city  are 
classified  as  commercial,  industrial,  or  residential.  The  residential 
districts  can  be  subdivided  into  sparsely  populated,  moderately 
populated,  crowded,  wealthy,  poor,  etc.  Commercial  districts 
may  be  either  retail  stores,  office  buildings,  or  wholesale  houses. 
Industrial  districts  may  be  either  large  factories,  foundries,  etc., 
or  they  may  be  made  up  of  small  industries  housed  in  loft  build- 
ings. 

In  cities  of  less  than  30,000  population  the  refinement  of  such 
subdivisions  is  generally  unnecessary  in  the  study  of  sewage  flow, 
all  districts  being  considered  the  same.  The  data  given  in  Tables 
8  and  9  indicate  the  difference  to  be  found  in  different  districts  of 


CHARACTER  OF  DISTRICT 


33 


large  cities.  The  Milwaukee  data  are  presented  in  a  form  avail- 
able for  estimates  on  different  bases.  These  data  are  shown  in 
Table  10. 

TABLE  7 
RATES  OF  WATER  CONSUMPTION 

From  Journals  of  American  and  New  England  Water  Works  Associations 


Con- 

Con- 

Popu- 

sump- 

Popu- 

sump- 

lation 

Per 

tion, 

lation 

Per 

tion, 

City 

in 

Cent 

Gal. 

City 

in 

Cent 

Gal. 

Thou- 

Metered 

per 

Thou- 

Metered 

per 

sands 

Capita 

sands 

Capita 

per  Day 

per  Day 

Tacoma,  Wash.  .... 

100 

11.6 

460 

Jefferson  City,  Mo  . 

13.5 

34.4 

100 

Buffalo,  N.  Y  

450 

4.9 

310 

^luncie    Ind 

30 

23.8 

95 

Chevenne   ^Vyo  .... 

13 

270 

Burlington   la 

24 

4.5 

90 

Erie    Pa                .... 

72 

3.0 

198 

Council  Bluffs,  la  .  . 

32 

75.5 

80 

Philadelphia,  Pa  

1611 

4.6 

180 

San  Diego,  Cal  .... 

85 

100 

80 

St.  Catherines,  Ont. 

17 

3.2 

160 

Monroe,  Wis  

3 

100 

80 

Port  Arthur,  Ont... 

18 

14.7 

145 

Yazoo  City,  Miss  .  . 

7 

84.1 

75 

Ogdensburg,  N.  Y  .  . 

18 

0.2 

140 

Oak  Park,  Illinois  .  . 

26 

100 

70 

Los  Angeles,  Cal.  .  .  . 

516 

77.9 

140 

Portsmouth,  Va.  .  .  . 

75 

8.1 

65 

Wilmington,  Del.  .  . 

92 

43.7 

125 

New  Orleans,  La.  .  . 

360 

99.7 

60 

Lancaster,  Pa  

60 

34.6 

120 

Rockford,  111  

53 

93.0 

55 

Richmond,  Va  

120 

75.2 

115 

Fort  Dodge,  la  .... 

20 

96.0 

50 

St.  Louis,  Mo  

730 

6.7 

110 

Manchester,  Vt  

1.5 

69.0 

45 

Springfield,  Mass.  .  . 

100 

94.4 

110 

Woonsocket,  R.  I  .  . 

47.5 

95.6 

35 

Keokuk    la 

14 

64.5 

105 

Attempts  have  been  made  to  express  the  rate  of  sewage  flow 
in  different  units  other  than  in  gallons  per  capita  per  day.  A  unit 
in  terms  of  gallons  per  square  foot  of  floor  area  tributary  has  been 
suggested  for  commercial  and  industrial  districts.  It  has  not 
been  generally  adopted.  The  rates  of  flow  in  New  York  City  as 
reported  in  this  unit  by  W.  S.  McGrane  are  given  in  Table  11. 

The  most  successful  way  to  predict  the  flow  from  commercial 
or  industrial  districts  is  to  study  the  character  of  the  district's 
activities  and  to  base  the  prediction  on  the  quantity  of  water 
demanded  by  the  commerce  and  industry  of  the  district  affected. 

25.  Fluctuations  in  Rate  of  Sewage  Flow. — The  rate  of  flow 
of  sewage  from  any  district  varies  with  the  season  of  the  year, 
the  day  of  the  week,  and  the  hour  of  the  day.  The  maximum 
and  minimum  rates  of  sewage  flow  are  the  controlling  factors  in 
the  design  of  sewers.  The  sewers  must  be  of  sufficient  capacity 


34 


QUANTITY  OF  SEWAGE 


to  carry  the  maximum  load  which  may  be  put  upon  them,  and 
they  must  be  on  such  a  grade  that  deposits  will  not  occur  during 
periods  of  minimum  flow.  The  maximum  and  minimum  rates  of 
flow  are  usually  expressed  as  percentages  of  the  average  rate  of 
flow. 

TABLE  8 
SEWAGE  FLOW  FROM  DIFFERENT  CLASSES  OF  DISTRICTS 

Arranged  from  data  by  Kenneth  Allen  in  Municipal  Engineer's  Journal, 
Feb.,  1918. 


District 


Gallons 

per 

Capita 
per  Day 


Gallons 

per 

Acre 
per  Day 


Buffalo,  N.  Y.    From  Report  of  International  Joint  Com- 
mission on  the  Pollution  of  Boundary  Waters: 

Industrial:  Metal  and  automobile  plants.     Maximum 13,000 

Industrial:  Meat  packing,  chemical  and  soap  .* 16,000 

Commercial:  Hotels,  stores  and  office  buildings 60,000 

Domestic :  Average 80 

Domestic :  Apartment  houses 147 

Domestic:  First-class  dwellings 129 

Domestic:  Middle-class  dwellings 81 

Domestic :  Lowest-class  dwellings 35 . 5 

Cincinnati,  Ohio.    1913  Report  on  Sewerage  Plan: 

Industrial,  in  addition  to  residential  and  ground  water 9,000 

Commercial,  in  addition  to  residential  and  ground  water       ....         40,000 
Domestic 135 

Detroit,  Mich.: 

Domestic 228 

Industrial,  in  addition  to  residential  and  ground  water 12,000 

Commercial,  in  addition  to  residential  and  ground  water       ....         50,000 

Milwaukee/  Wis.     1915  Report  of  Sewerage  Commission: 

Industrial,  maximum 81  16,600 

Industrial,  average 31  8,300 

Commercial,  maximum 60,500 

Commercial,  average 37,400 

Wholesale  commercial,  maximum 20,000 

Wholesale  commercial,  average 9,650 


CHARACTER  OF   DISTRICT 


35 


TABLE  9 

OBSERVED  WATER  CONSUMPTION  IN  DIFFERENT  CLASSES  OF  DISTRICTS  IN 

NEW  YORK  CITY 

From  data  by  Kenneth  Allen  in  Municipal  Engineers  Journal,  for  1918 


Daily 

Daily 

Daily 

Cons. 

Cons. 

Cons. 

Hotels 

Gals,  per 
1000 
Sq.  Ft. 

Tenements 

Gals,  per 
1000 
Sq.  Ft. 

Office  and  Loft 
Buildings 

Gals,  per 
1000 
Sq.  Ft. 

Floor 

Floor 

Floor 

Area 

Area 

Area 

Building 

* 

X 
03 

> 

Location 

X 

a 

K* 

Building 

* 

03 

M 
> 

Hotel  Biltmore.  .  .  . 

470 

368 

78th-79th  St.  and 

McGraw  Bldg  

309 

206 

Hotel  McAlpin.  .  .  . 

753 

694 

B'way  

256 

192 

N.     Y.    Telephone 

Hotel  Pla/a.  ...... 

630 

578 

410  E.  65th  St  

350 

295 

Bldg  

194 

Hotel    Waldorf 

30th  St.  and  Madi- 

Met. Life  Bldg..  . 

256 

Astoria  

618 

482 

son  Ave  

306 

188 

42d  St.  Bldg  

271 

Hotel  Astor  

732 

492 

27  Lewis  St  

307 

250 

Municipal  Bldg.  .  . 

118 

Hotel  Vanderbilt  .  . 

604 

545 

258  Delancey  St  .  . 

267 

226 

Equitable  Bldg  

366 

268 

Average  

634 

526 

Average  

297 

230 

Average  338 

219 

*  Max.  represents  only  the  average  maximum,  not  the  greatest  maximum. 

TABLE  10 

SEWAGE  FLOW  FROM  DIFFERENT  CLASSES  OF  DISTRICTS  BASED  ON   1915 
REPORT  OF  MILWAUKEE  SEWERAGE  COMMISSION 


Ratio  of  maximum  to  average  rate  for  department  store  district 

Ratio  of  maximum  to  average  rate  for  hotel  district 

Ratio  of  maximum  to  average  rate  for  office  building  district 

Ratio  of  maximum  to  average  rate  for  wholesale  commercial  district. 


1.755 
1.65 
1.51 
2.1 


Average  and  maximum  gallons  per  thousand  square  feet  of 

Avg. 

Max. 

n 

noor  area  . 
For  department  store  district  

232 

407 

For  office  building  district  

541 

891 

For  wholesale  commercial  district 

164 

344 

For  all  districts  except  wholesale  commercial  

381 

618 

Average  and  maximum  gallons  per  day  : 

For  all  districts  except  wholesale  commercial  

17,700 

29,800 

For  wholesale  commercial  district  

9,650 

20,000 

36 


QUANTITY  OF  SEWAGE 


TABLE  11 

RATES  OF  CONSUMPTION  PREDICTED  FOR  DIFFERENT  DISTRICTS  IN  NEW  YORK 

CITY 


£.8 

is 

4* 

o 

£ 

O 

o< 

I 

I 

£J 

§ 

M<^ 

S<i 

3 

0"< 

o3  ^ 
«  o 

5 

DO 

>J 

£t 

d+J 

.Sg 

0 

O 

.si 

1 

P"; 

sj 

aj 

District 

eS  t,  (S 

3.1 

1 

3d- 

60 

en    . 

C  o* 

oS 

1 

II 

l! 

|| 

?| 

|| 

III 

a 

»§ 

•g| 

T3 

III 

3! 

«l 

ll 

ll 

"o 

"w  ^ 

4)   W*J 

bi 
> 

j& 

11 

'•3s  « 

<u^!  P. 

%l 

ill 

i^-5 

* 

o 

O 

£ 

£ 

£ 

£ 

S 

Hotel  and  midtown 

24,800 

15 

634 

526 

500 

.20 

.29 

.34 

1.04 

.146 

Midtown  and  financial 

24,800 

15 

338 

219 

300 

.12 

.  18 

.23 

.078 

.110 

East  and  West  of  midtown.  .  .  . 

24,800 

10 

297 

230 

300 

.074 

.12 

.15 

.057 

.097 

Apartment,  59th  to  155th  Sts  .  . 

20,400 

7 

230 

300 

.043 

.06 

.09 

Manhattan  north  of  155th  St  .  . 

20,400 

5 

230 

300 

.031 

.05 

.08 

Midtown  district  consists  of  department  stores,  large  railroad  terminals,  industrial  and 
loft  buildings,  and  sky-scraper  office  buildings. 

It  is  difficult  to  set  any  definite  figure  for  the  percentage  which 
the  maximum  rate  of  flow  is  of  the  average.  Fluctuations  above 
and  below  the  average  are  greater  the  smaller  the  tributary 
population.  This  relation  can  be  expressed  empirically  as 

,,     500 


in  which  M  represents  the  per  cent  which  the  maximum  flow  is 
of  the  average,  and  P  represents  the  tributary  population  in 
thousands.  The  expression  should  not  be  used  for  populations 
below  1,000  nor  above  1,000,000.  Having  determined  'the  expected 
average  flow  of  sewage  by  a  study  of  the  population,  water  con- 
sumption, etc.,  the  maximum  quantity  of  sewage  is  determined 
by  multiplying  the  average  flow  by  the  per  cent  which  the  maxi- 
mum is  of  the  average.  In  this  connection  W.  G.  Harmon1  offers 
the  relation 


, 

4+VP' 

which  was  used  in  the  design  of  the  Ten  Mile  Creek  intercepting 

sewer  at  Toledo,  Ohio.     For  rough  estimates  and  for  comparative 

purposes  the  ratio  of  the  average  to  the  minimum  flow  can  be 

1  Eng.  News-Record,  Vol.  80,  page  1233,  1918. 


FLUCTUATIONS  IN  RATE  OF  SEWAGE  FLOW 


37 


taken  the  same  as  the  ratio  of  the  maximum  to  the  average  flow, 

unless  direct  gaugings  or  other  information  show  it  to  be  otherwise. 

The  fluctuations  of  flow  in  commercial  and  industrial  districts 

are  so  different  from  those  in  residential  districts  that  the  formulas 


I    23456789   10 
A.M. 


123456789   10 
P.M. 


FIG.  10. — Daily  and  Hourly  Variations  of  Sewage  Flow. 

1.  Toledo,  O.     Manufacturing  average.  6.  Toledo,  O.;    Residential,  Sunday. 


2.  Toledo,  O. 

3.  Toledo,  O. 


Manufacturing,  Monday. 
Manufacturing,  Sunday. 


4.  Toledo,  O.     Residential,  average. 

5.  Toledo,  O.;    Residential,  Monday. 


7.  Cincinnati,  O.,    Industrial,  average. 

8.  Cincinnati,  O.;    Residential,  average. 

9.  Cincinnati,  O.;  Commercial,  average. 
10.  Average  of  7  cities. 


given  should  not  be  used  in  the  design  of  sewers  other  than  those 
draining  residential  areas.  It  is  reasonable  to  suppose  that 
fluctuations  in  rates  of  flow  from  industrial  districts  are  dependent 


38  QUANTITY  OF  SEWAGE 

upon  the  character  of  the  tributary  industries.  A  study  of  these 
industries  will  give  valuable  light  on  the  maximum  and  minimum 
rates  at  which  sewage  will  be  delivered  to  the  sewers. 

Hourly,  daily,  and  seasonal  fluctuations  in  rates  of  sewage 
flow  are  of  interest  in  the  design  of  pumping  stations  to  give 
knowledge  of  the  rates  at  which  the  pumps  must  operate  at 
various  periods.  The  fluctuations  in  rates  of  sewage  flow  during 
various  hours  and  days  in  different  cities  and  districts  are  shown 
in  Fig.  10.  Fluctuations  in  rate  of  flow  of  sewage  lag  behind 
fluctuations  in  rate  of  water  consumption,  the  time  being  depend- 
ent on  the  distance  through  which  the  wave  of  change  must 
travel  in  the  sewer. 

26.  Effect  of  Ground  Water. — Sewers  are  seldom  laid  with 
water-tight   joints.     Since   they   usually   lie   below   the   ground 
water  level  it  is  inevitable  that  a  certain  amount  of  ground  water 
will  enter.     Various  units  have  been  suggested  for  the  expression 
of  the  inflow  of  ground  water  in  an  attempt  to  include  all  of  the 
many  factors.     Some  of  these  units  are:  gallons  per  acre  drained 
by  the  sewer  per  day,  gallons  per  mile  of  pipe  per  day,  gallons  per 
inch  diameter  per  mile  of  pipe  per  day,  etc.     Since  the  ground 
water  enters  pipe  sewers  at  the  joints,  the  longer  the  joints  the 
greater  the  probability  of  the  entrance  of  ground  water.     The 
last  unit  is  therefore  the  most  logical  but  the  accuracy  of  the 
result  is  scarcely  worthy  of  such  refinement  and  the  unit  usually 
adopted  is  gallons  per  mile  of  pipe  per  day. 

No  definite  figure  can  be  given  for  the  amount  of  ground 
water  to  be  expected  in  sewers  since  the  character  of  the  soil  and 
the  ground  water  pressure  must  be  considered.  Relatively 
normal  infiltration  may  be  found  from  5,000  to  80,000  gallons  per 
mile  of  pipe  per  day.  The  minimum  is  seldom  reached  in  wet 
ground  and  the  maximum  is  frequently  exceeded.  Table  12 
shows  the  amount  of  ground  water  measured  in  various  sewers 
as  given  by  Brooks.1 

27.  Resume  of  Method  for  Determination  of  Quantity  of  Dry- 
weather  Sewage. — The  steps  in  the  determination  of  the  quantity 
of  sewage  are:   determine  the  period  in  the  future  for  which  the 
sewers  are  to  be  designed;   estimate  the  population  and  tributary- 
area  at  the  end  of  this  period;  estimate  the  rate  of  water  consump- 

1  Infiltration  of  Ground  Water  into  Sewers.  Transactions  of  the  American 
Society  of  Civil  Engineers,  Vol.  76, 1913,  p.  1909. 


EFFECT  OF  GROUND  WATER 


39 


S    2 


pq    o 

g  | 


s^ 

OJ  <<-< 


o 


Ifcijjrf 

;?    S's    ^ 


J 


r. 


i>o  •  -o  o  o  oo 

O  O5  •  •  O  i— i  ^F  OS  00 

<M  00  •  •  i— i  O  tO  00  Tf 

IN"  :  :  OO~I-<"<N~I-H~IO' 


•  O  O  CO 

•  O5  C^  (N 


CO  O      • 

d»o    • 


t.?  |>  t^.  Oi 

•i-irji  oir^ 


:  :PQ 
-8 


-  - 


PL,     .pi,     .     .  .pH     . 

:>: 


: 


40  QUANTITY  OF  SEWAGE 

tion  and  assume  the  sewage  flow  to  equal  the  water  consumption; 
determine  the  maximum  and  minimum  rates  of  sewage  flow; 
and  finally,  estimate  the  maximum  rate  of  ground  water  seepage 
and  add  it  to  the  maximum  rate  of  sewage  flow  to  give  the  total 
quantity  of  sewage  to  be  carried  by  the  proposed  sewers. 

QUANTITY  OF  STORM  WATER 

28.  The  Rational  Method.— The  water  which  falls  during  a 
storm  must  be  removed  rapidly  in  order  to  prevent  the  flooding 
of  streets  and  basements,  and  other  damages.     The  quantity  of 
water  to  be  cared  for  is  dependent  upon:  the  rate  of  rainfall,  the 
character  and  slope  of  the  surface,  and  the  area  to  be  drained. 
All  methods  for  the  determination  of  storm  water  run-off,  whether 
rational  or  empirical,  depend  upon  these  factors. 

The  so-called  Rational  Method  can  be  expressed  algebraically, 
as, 

Q  =  AIR, 

in  which    Q  =  rate  of  run-off  in  cubic  feet  per  second; 
A  =  area  to  be  drained  expressed  in  acres; 
7  =  percentage  imperviousness  of  the  area; 
R  —  maximum  average  rate  of  rainfall  over  the  entire 
drainage  area,  expressed  in  inches  per  hour,  which 
may  occur  during  the  time  of  concentration. 

The  area  to  be  drained  is  determined  by  a  survey.  A  discussion 
of  R  and  7  follows  in  the  next  two  sections.  An  example  of  the 
use  of  the  Rational  Method  is  given  on  page  95. 

29.  Rate  of  Rainfall. — Rainfall  observations  have  been  made 
over  a  long  period  of  time  by  United  States  Weather  Bureau 
observers  and  others.     Continuous  records  are  available  in  a  few 
places   in   this   country   showing   rainfall   observations   covering 
more  than  a  century.     Such  records  have  been  the  bases  for  a 
number  of  empirical  formulas  for  expressing  the  probable  maximum 
rate  of  rainfall  in  inches  per  hour,  having  given  the  duration  of 
the  storm.     Table  13  is  a  collection  of  these  formulas  with  a 
statement  as  to  the  conditions  under  which  each  formula  is  appli- 
cable.    The  formula  most  suitable  to  the  problem  in  hand  should 
be  selected  for  its  solution.1 

1  A  comprehensive  discussion  of  rainfall  formulas  will  be  found  in  Vol.  54 
of  the  Transactions  Am.  Society  of  Civil  Engineers,  1905. 


RATE  OF  RAINFALL 


41 


TABLE  13 
RAINFALL  FORMULAS 


Name  of 
Originator 

Conditions  for  which  Formula  is 
Suitable 

Formula 

E  S  Dorr 

150 

A.  N.  Talbot.  .  .. 
A.  N.Talbot  

Emil  Kuichling.  .  . 
L.  J.  LeConte.... 

Sherman  
Sherman  
Webster  
Hendrick 

Maximum   storms  in   Eastern   United 
States  
Ordinary    storms    in    Eastern    United 
States 

J+30 
.      360 

105 

2-  — 

120 
i=  ,  etc. 
Z+20' 

105 

Heavy  rainfall  near  New  York  City.  . 

For  San  Francisco.    See  T.  A.  S.  C.  E. 
v  54  p  198 

Maximum  for  Boston,  Mass  

Extraordinary  for  Boston,  Mass  
Ordinary  for  Philadelphia,  Pa 

Ordinary  storms  for  Baltimore.     Eng. 
&  Cont.,  Aug.  9.  1911  

J.  de  Bruyn-Kops. 

C  D.  Hill  

Metcalf  and  Eddy 
W.W.  Horner.... 

R.A  Brackenbu} 

Metcalf  and  Eddy 
Metcalf  and  Eddy 
Kenneth  Allen  .  .  . 

.      163 

Ordinary  storms  for  Savannah,  Ga  .  .  .  . 
For  Chicago  111 

t+27 
120 

%  "— 

Louisville,  Ky.    Am.  Sew.  Prac.,  Vol  I. 
St.  Louis,  Mo.    Eng.  News,  Sept.  29, 
1910             

For  Spokane,  Wash.   Eng.  Record,  Aug. 
10  1912 

84 
400    * 

New  Orleans              

For  Denver,  Colo  

Central  Park,  N.  Y     51-Year  Record. 
Eng.    News-Record,    April    7,    192  1, 
p  588                          

2J+40 

*  Formula  devised  by  H.  E.  Babbitt  from  Allen's  25-year  curve. 

30.  Time  of  Concentration. — By  the  time  of  concentration  is 

meant  the  longest  time  without  unreasonable  delay  that  will  be 

required  for  a  drop  of  water  1  to  flow  from  the  upper  limit  of  a 

drainage  area  to  the  outlet.     Assuming  a  rainfall  to  start  sud- 

1  See  Note  under  Table  14. 


42  QUANTITY  OF  SEWAGE 

denly  and  to  continue  at  a  constant  rate  and  to  be  evenly  dis- 
tributed over  a  drainage  area  of  100  per  cent  imperviousness  and 
even  slope  towards  one  point,  the  rate  of  run-off  would  increase 
constantly  until  the  drop  of  water  from  the  upper  limit  of  the  area 
reached  the  outlet,  after  which  the  rate  of  run-off  would  remain 
constant.  In  nature  the  rate  of  rainfall  is  not  constant.  The 
shorter  the  duration  of  a  storm  the  greater  the  intensity  of  rain- 
fall. Therefore  the  maximum  run-off  during  a  storm  will  occur 
at  the  moment  when  the  upper  limit  of  the  area  has  commenced 
to  contribute.  From  that  time  on  the  rate  of  run-off  will  decrease. 
The  time  of  concentration  can  be  measured  fairly  well  by 
observing  the  moment  of  the  commencement  of  a  rainfall,  and  the 
time  of  maximum  run-off  from  an  area  on  which  the  rain  is  falling. 
A  prediction  of  the  time  of  concentration  is  more  or  less  guess 
work.  As  the  result  of  measurements  some  engineers  assume  the 
time  of  concentration  on  a  city  block  built  up  with  impervious 
roofs  and  walks,  and  on  a  moderate  slope,  is  about  5  to  10  minutes. 
This  is  used  as  a  basis  for  the  judgment  of  the  time  of  concentra- 
tion on  other  areas.  For  relatively  large  drainage  areas  such  a 
method  cannot  be  used.  The  procedure  is  to  measure  the  length 
of  flow  through  the  drainage  channels  of  the  area,,  to  assume  the 
velocity  of  the  flood  crest  through  these  channels  and  thus  to 
determine  the  time  of  concentration.  Table  14  shows  the  flood 
crest  velocities  in  various  streams  of  the  Ohio  River  Basin  under 
flood  conditions.  The  velocity  over  the  surface  of  the  ground 
may  be  approximated  by  the  use  of  the  formula1 

V  =  2,0007  VS, 

in  which    V  =  the  velocity  of  flow  over  the  surface  of  the  ground 

in  feet  per  minute; 

J  =  the  percentage  imperviousness  of  the  ground; 
$  =  the  slope  of  the  ground. 

For  areas  up  to  100  acres  where  natural  drainage  channels  are  not 
existent  this  formula  will  give  more  satisfactory  results  than  guesses 
based  on  the  time  of  concentration  of  certain  known  areas. 

Having  determined  the  time  of  concentration,  the  rate  of  rain- 
fall R  to  be  used  in  the  Rational  Method  is  found  by  substitution 
in  some  one  of  the  rainfall  formulas  given  in  Table  13. 
1  Sewerage  by  A.  P.  Folwell. 


TIME  OF  CONCENTRATION 


43 


s  s 
si 


PQ  a 
«& 


D  CREST  VELOCITI 
From  Table  12,  U. 


F 


- 

S-S.9  g 


•+-1    JH    f-t    02          ^ 

iplii 


^>     c«r 

±2  $  3 


O^^fM^r-l        OO5C<IOT^QOiC«OrHcOrt<i-tCOO 

^-1  T-H  rH  ^H          rH          i—  1          i—  1  O5          Tt*  t-l  r-(          CQ  i—  1  »-H          rH 


O<MOOCOt^r-tpOC^O5COCOcO<N 
ail>'1*»OiO-<tfiOiO>O»OiO»OTt<-^ 


T-iO      •  lO  CO  -H  I-H  O5  (M  iM  CO  CO  CO  C<1  rt<  rt<  iO  (N  CO 


S 


C^OOrH 


QO  b-  l-  r^OO  O  i 


8(N(N 
00  i—  i 


CO  QO  CO  O  -^  O  ^H 


53   03 


f<      j- 

s-^i^"ssisij 

^O-3     r-d3'c3^^3  .rO     ro^ 


ity 
ver 


he 
f 


i' 


**  2* 

i; 


SI 

II 


*£ 

—  o 


*"^ 

"o** 

S2 

s:« 


ie 
in 


>•  P 
JS^d 

1U 
HS 

|«'C 

1M 

•2-g.g 

Us 


•«i 

•sJS 


44  QUANTITY  OF  SEWAGE 

31.  Character  of  Surface. — The  proportion  of  total  rainfall 
which  will  reach  the  sewers  depends  on  the  relative  porosity,  or 
imperviousness,  and  the  slope  of  the  surface.  Absolutely  impervi- 
ous surfaces  such  as  asphalt  pavements  or  roofs  of  buildings  will 
give  nearly  100  per  cent  run-off  regardless  of  the  slope,  after  the 
surfaces  have  become  thoroughly  wet.  For  unpaved  streets, 
lawns,  and  gardens  the  steeper  the  slope  the  greater  the  per  cent 
of  run-off.  When  the  ground  is  already  water  soaked  or  is  frozen 
the  per  cent  of  run-off  is  high,  and  in  the  event  of  a  warm  rain  on 
snow  covered  or  frozen  ground,  the  run-off  may  be  greater  than  the 
rainfall.  The  run-off  during  the  flood  of  March,  1913, at  Columbus, 
Ohio,  was  over  100  per  cent  of  the  rainfall.  Table  15  l  shows  the 
relative  imperviousness  of  various  types  of  surfaces  when  dry 
and  on  low  slopes.  The  estimates  for  relative  impervious- 
ness  used  in  the  design  of  the  Cincinnati  intercepter  are  given  in 
Table  16. 

TABLE  15 
VALUES  OF  RELATIVE  IMPERVIOUSNESS 

Roof  surfaces  assumed  to  be  watertight 0 . 70-0 . 95 

Asphalt  pavements  in  good  order 85-  .  90 

Stone,  brick,  and  wood-block  pavements  with  tightly  cemented 

joints 75-  .  85 

The  same  with  open  or  ancemented  joints 50-  .  70 

Inferior  block  pavements  with  open  joints 40-  .  50 

Macadamized  roadways 25-  .  60 

Gravel  roadways  and  walks 15-  .  30 

Unpaved  surfaces,  railroad  yards,  and  vacant  lots 10-  .  30 

Parks,  gardens,  lawns,  and  meadows,  depending  on  surface  slope 

and  character  of  subsoil 05-  .  25 

Wooded  areas  or  forest  land,  depending  on  surface  slope  and  char- 
acter of  subsoil 01-  .20 

Most  densely  populated  or  built  up  portion  of  a  city 70-  .90 

C.  E.  Gregory  2  states  that  /,  in  the  expression  Q  =  AIR  is  a 
function  of  the  time  of  concentration  or  the  duration  of  the  storm. 
If  t  represents  the  time  of  concentration  and  T  represents  the 
duration  of  the  storm,  then  when  T  is  less  than  t 

7=0.175**, 

1  From  an  article  by  E.  Kuichling  in  Transactions  American  Society  of 
Civil  Engineers,  Vol.  65,  1909,  p.  399. 

2  Trans.  Am.  Society  Civil  Engineers,  Vol.  58,  1907,  p.  483. 


CHARACTER  OF  SURFACE 


45 


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46  QUANTITY  OF  SEWAGE 

but  when  T  is  greater  than  t, 


Gregory  condenses  Kuichling's  rules  with  regard  to  the  per  cent 
run-off,  as  follows: 

1.  The  per  cent  of  rainfall  discharged  from  any  given 
drainage  area  is  nearly  constant  for  heavy  rains  lasting 
equal  periods  of  time. 

2.  This  per  cent  varies  directly  with  the  area  of  imper- 
vious surface. 

3.  This  per  cent  increases  rapidly  and  directly  or  uni- 
formly with  the  duration  of  the  maximum  intensity  of  the 
rainfall  until  a  period  is  reached  which  is  equal  to  the  time 
required  for  the  concentration  of  the  drainage  waters  from 
the  entire  area  at  the  point  of  observation,  but  if  the  rain- 
fall continues  at  the  same  intensity  for  a  longer  period  this 
per  cent  will  continue  to  increase  at  a  much  smaller  rate. 

4.  This  per  cent  becomes  larger  when  a  moderate  rain 
has  immediately  preceded  a  heavy  shower  on  a  partially 
permeable  territory. 

Gregory's  formulas  have  not  been  generally  accepted  and  are 
not  widely  used  in  practice.  Marston  stated  :  l 

All  that  engineers  are  at  present,  warranted  in  doing  is 
to  make  some  deduction  from  100  per  cent  run-off  .  .  .  the 
deduction  .  .  .  being  at  present  left  to  the  engineer  in 
view  of  his  general  knowledge  and  his  familiarity  with  local 
conditions. 

Burger  states  2  in  the  same  connection  : 

In  its  application  there  will  usually  be  as  many  results 
(differing  widely  from  each  other)  as  the  number  of  men 
using  it. 

In  spite  of  these  objections  the  Rational  Method  is  in  more  favor 
with  engineers  than  any  other  method. 

32.  Empirical  Formulas.  —  The  difficulty  of  determining  run- 
off with  accuracy  has  led  to  the  production  by  engineers  of  many 
empirical  formulas  for  their  own  use.  Some  of  these  formulas 
have  attracted  wide  attention  and  have  been  used  extensively, 

1  Trans.  American  Society  of  Civil  Engineers,  Vol.  58,  1907,  p.  498. 

2  Ibid. 


EMPIRICAL  FORMULAS  47 

in  some  cases  under  conditions  to  which  they  are  not  applicable. 
In  general  these  formulas  are  expressions  for  the  run-off  in  terms 
of  the  area  drained,  the  relative  imperviousness,  the  slope  of  the 
land,  and*  the  rate  of  rainfall. 

The  Burkli-Ziegler  formula,  devised  by  a  Swiss  engineer  for 
Swiss  conditions  and  introduced  into  the  United  States  by  Rudolph 
Hering,  was  one  of  the  earliest  of  the  empirical  formulas  to  attract 
attention  in  this  country.  It  has  been  used  extensively  in  the 
form 


Q-CM-Jg, 


in  which    Q  =  the  run-off  in  cubic  feet  per  second; 

i  =  the  maximum  rate  of  rainfall  in  inches  per  hour  over 
the  entire  area.  This  is  determined  only  by  ex- 
perience in  the  particular  locality,  and  is  usually 
taken  at  from  1  to  3  inches  per  hour; 

£  =  the  slope  of  the  ground  surface  in  feet  per  thousand, 

A  =  the  area  in  acres; 

C  =  an  expression  for  the  character  of  the  ground  sur- 
face, or  relative  imperviousness.  In  this  form  of 
the  expression  C  is  recommended  as  0.7. 

The  McMath  formula  was  developed  for  St.  Louis  conditions 
and  was  first  published  in  Transactions  of  the  American  Society 
of  Civil  Engineers,  Vol.  16, 1887,  p.  183.  Using  the  same  notation 
as  above,  the  formula  is, 


McMath  recommended  the  use  of  C  equal  to  0.75,  i  as  2.75  inches 
per  hour,  and  S  equal  to  15*.  The  formula  has  been  extended 
for  use  with  all  values  of  C,  i,  S,  and  A  ordinarily  met  in  sewerage 
practice.  Fig.  11  is  presented  as  an  aid  to  the  rapid  solution  of 
the  formula. 

Other  formulas  have  been  devised  which  are  more  applicable 
to  drainage  areas  of  more  than  1,000  acres.1  Such  areas  are  met 
in  the  design  of  sewers  to  enclose  existing  stream  channels  drain- 
ing large  areas.  Kuichling's  formulas,  published  in  1901  in  the 

1  The  principles  governing  the  run-off  from  large  areas  are  explained  in 
Elements  of  Hydrology,  by  A.  F.  Meyer,  1917. 


QUANTITY  OF  SEWAGE 


r 


:20 

-30 
-40 

^60 

^80 

100 


200 

^300 

-400 

500 

^600 

iisoo 

^1000 


3000 
-4000 

5000 
:6000 

^8000 
:.10,000 


Values  of  c 
«0.1        0.2  0.30.40.5 


0.7  1.0 


FIG.  11. — Diagram  for  the  Solution  of  McMath's  Formula, 


EMPIRICAL  FORMULAS 


49 


report  of  the  New  York  State  Barge  Canal,  were  devised  for  areas 
greater  than  100  square  miles.  The  following  modification  of 
these  formulas  for  ordinary  storms  on  smaller  areas  was  published 
for  the  first  time  in  American  Sewerage  Practice,  Volume  I,  by 
Metcalf  and  Eddy : 

25000 

+ 


30          40  50          60          70 

Quantity  in  Cubic  Feet  per  Second. 

FIG.  12. — Comparison  of  Empirical  Run-off  Formulas. 

It  is  to  be  noted  that  the  only  factor  taken  into  consideration  is 
the  area  of  the  watershed.  It  is  obvious  that  other  factors  such 
as  the  rate  of  rainfall,  slope,  imperviousness,  etc.,  will  have  a 
marked  effect  on  the  run-off. 

There  are  other  run-off  formulas  devised  for  particular  con- 
ditions, some  of  which  are  of  as  general  applicability  as  those 
quoted.  Two  formulas  which  are  frequently  quoted  are:  Fan- 
ning's,  Q  =  2QQM5/*  and  Talbot's  Q  =  500MM,  in  which  M  is  the  area 
of  the  watershed  in  square  miles.  A  comprehensive  treatment 
of  the  subject  is  given  in  American  Sewerage  Practice,  Vol.  I, 
by  Metcalf  and  Eddy. 

A  comparison  of  the  results  obtained  by  the  application  of  a 
few  formulas  to  the  same  conditions  is  shown  graphically  in  Fig. 
12.  It  is  to  be  noted  that  the  divergence  between  the  smallest 


50  QUANTITY  OF  SEWAGE 

and  largest  results  is  over  100  per  cent.  As  these  formulas  are 
not  all  applicable  to  the  same  conditions,  the  differences  shown  are 
due  partially  to  an  extension  of  some  of  them  beyond  the  limits 
for  which  they  were  prepared. 

33.  Extent  and  Intensity  of  Storms. — In  the  design  of  storm 
sewers  it  is  necessary  to  decide  how  heavy  a  storm  must  be  pro- 
vided for.  The  very  heaviest  storms  occur  infrequently.  To 
build  a  sewer  capable  of  (faring  for  all  storms  would  involve  a 
prohibitive  expense  over  the  investment  necessary  to  care  for  the 
ordinary  heavy  storms  encountered  annually  or  once  in  a  decade. 
This  extra  investment  would  lie  idle  for  a  long  period  entailing  a 
considerable  interest  charge  for  which  no  return  is  easily  seen. 
The  alternative  is  to  construct  only  for  such  heavy  storms  as  are 
of  ordinary  occurrence  and  to  allow  the  sewers  to  overflow  on 
exceptional  occasions.  The  result  will  be  a  more  frequent  use  of 
the  sewerage  system  to  its  capacity,  a  saving  in  the  cost  of  the 
system,  and  an  occasional  flooding  of  the  district  in  excessive 
storms.  The  amount  of  damage  caused  by  inundations  must  be 
balanced  against  the  extra  cost  of  a  sewerage  system  to  avoid  the 
damage.  A  municipality  which  does  not  provide  adequate 
storm  drainage  is  liable,  under  certain  circumstances,  for  damages 
occasioned  by  this  neglect.  It  is  not  liable  if  no  drainage  exists, 
nor  is  it  liable  if  the  storm  is  of  such  unusual  character  as  to  be 
classed  legally  as  an  act  of  God. 

Kuichling's  studies  of  the  probabilities  of  the  occurrence  of 
heavy  storms  are  published  in  Transactions  of  the  American 
Society  of  Civil  Engineers,  Vol.  54,  1905,  p.  192.  Information 
on  the  extent  of  rain  storms  is  given  by  Francis  in  Vol.  7,  1878, 
p.  224,  of  the  same  publication.  Kuichling  expresses  the  intensity 
of  storms  which  will  occur, 

.     in  .      105 

once  in  10  years  as  i  •• 


.     1C  .120 

once  in  15  years  as  i  = 


in  which  i  is  the  intensity  of  rainfall  in  inches  per  hour  and  t  is 
the  duration  of  the  storm  in  minutes. 


CHAPTER  IV 
THE  HYDRAULICS  OF  SEWERS 

34.  Principles. — The  hydraulics  of  sewers  deals  with  the 
application  of  the  laws  of  hydraulics  to  the  flow  of  water  through 
conduits  and  open  channels.  In  so  far  as  its  hydraulic  proper- 
ties are  concerned  the  characteristics  of  sewage  are  so  similar  to 
those  of  water  that  the  same  physical  laws  are  applicable  to  both. 
In  general  it  is  assumed  that  the  energy  lost  due  to  friction  between 
the  liquid  and  the  sides  of  the  channel  varies  as  some  function  of 
the  velocity,  usually  the  square,  and  that  the  total  energy  passing 
any  section  of  the  stream  differs  from  the  energy  passing  any 
other  section  only  by  the  loss  of  energy  due  to  friction. 

The  general  expression  for  the  flow  of  sewage  would  then  be, 

h=(f)Vn, 

in  which  h  is  the  head  or  energy  lost  between  any  two  sections, 
and  V  is  the  average  velocity  of  flow  between  these  sections. 
It  is  to  be  noted  in  this  general  expression  that  the  quantity  and 
rate  of  flow  past  all  sections  is  assumed  to  be  constant.  This 
condition  is  known  as  steady  flow.  Problems  are  encountered 
in  sewerage  design  which  involve  conditions  of  unsteady  flow, 
and  methods  of  solution  of  them  have  been  developed  based  on 
modifications  of  this  general  expression.  The  average  velocity 
of  flow  is  computed  by  dividing  the  rate  (quantity)  of  flow  past 
any  section  by  the  cross-sectional  area  of  the  stream  at  that 
section.  This  does  not  represent  the  true  velocity  at  any  par- 
ticular point  in  the  stream,  as  the  velocity  near  the  center  is  faster 
than  that  near  the  sides  of  the  channel.  The  distribution  of 
velocities  in  a  closed  circular  channel  is  somewhat  in  the  form  of 
a  paraboloid  superimposed  on  a  cylinder. 

The  laws  of  flow  are  expressed  as  formulas  the  constants  of 
which  have  been  determined  by  experiment.  It  has  been  found 
that  these  constants  depend  on  the  character  of  the  material 

51 


52  THE  HYDRAULICS  OF  SEWERS 

forming  the  channel  and  the  hydraulic  radius.  The  hydraulic 
ladius  is  denned  as  the  ratio  of  the  cross-sectional  area  of  the 
stream  to  the  length  of  the  wetted  perimeter,  or  line  of  contact 
between  the  liquid  and  the  channel,  exclusive  of  the  horizontal 
line  between  the  air  and  the  liquid. 

35.  Formulas.  —  The  loss  of  head  due  to  friction  caused  by 
flow  through  circular  pipes  flowing  full  as  expressed  by  Darcy  is, 


in  which  h  is  the  head  lost  due  to  friction  in  the  distance  /,  V  is 
the  velocity  of  flow,  g  is  the  acceleration  due  to  gravity,  and  /  is  a 
factor  dependent  on  d  and  the  material  of  which  the  pipe  is  made. 
A  formula  for  /  expressed  by  Darcy  as  the  result  of  experiments 
on  cast  iron  pipe  is, 


in  which  d  is  the  diameter  in  feet.     In  using  the  formula  with 
this  factor  the  units  used  must  be  feet  and  seconds. 

Another  form  of  the  same  expression  is  known  as  the  Chezy 
formula.  It  is  an  algebraic  transformation  of  the  Darcy  formula, 
but  in  the  form  shown  here,  by  the  use  of  the  hydraulic  radius, 
it  is  made  applicable  to  any  shape  of  conduit  either  full  or  partly 
full.  The  Chezy  formula  is, 

V  =  C\/RS, 

in  which  R  is  the  hydraulic  radius,  S  the  slope  ratio  of  the  hydraulic 
gradient,  and  C  a  factor  similar  to  /  in  the  Darcy  formula. 

Kutter's  formula  was  derived  by  the  Swiss  engineers,  Gan- 
guillet  and  Kutter,  as  the  result  of  a  series  of  experimental  observa- 
tions. It  was  introduced  into  the  United  States  by  Rudolph 
Hering  and  its  derivation  is  given  in  Hering  and  Trautwine's 
translation  of  "  The  Flow  of  Water  in  Open  Channels  by  Gan- 
guillet  and  Kutter."  In  English  units  it  is, 


V  = 


VRS, 


/        FORMULAS  53 

in  which  n  is  a  factor  expressing  the  character  of  the  surface  of 
the  conduit  and  the  other  notation  is  as  in  the  Chezy  formula. 
V  is  the  velocity  in  feet  per  second,  S  is  the  slope  ratio,  and  R  the 
hydraulic  radius  in  feet.  The  values  of  n  to  be  used  in  all  cases 
are  not  agreed  upon,  but  in  general  the  values  shown  below  are 
used  in  practice. 

VALUES  OF  n  IN  KUTTER'S  FORMULA 

n  CHARACTER  OF  THE  MATERIALS 

0 . 009  Well-planed  timber. 

0.010  Neat  cement  or  very  smooth  pipe. 

0.012  Unplaned  timber.     Best  concrete. 

0.013  Smooth  masonry  or  brickwork,  or  concrete 

sewers  under  ordinary  conditions. 

0.015  Vitrified  pipe  or  ordinary  brickwork. 

0.017  Rubble  masonry  or  rough  brickwork. 
0.020 


0  035  /      Smooth  earth 

0  050  }      R°ugh  channels  overgrown  with  grass. 

Kutter's  formula  is  of  general  application  to  all  classes  of  material 
and  to  all  shapes  of  conduits.  It  is  the  most  generally  used  for- 
mula in  sewerage  design. 

The  cumbersomeness  of  Kutter's  formula  is  caused  somewhat 
by  the  attempt  to  allow  for  the  effect  of  the  low  slopes  of  the 
Mississippi  River  experiments  on  the  coefficients.  The  correct- 
ness of  these  experiments  has  not  been  well  established  and  the 

0  0028 

slopes  are  so  flat  that  the  omission  of  the  term  -'—^ —  will  have 

o 

no  appreciable  effect  on  the  value  of  V  ordinarily  used  in  sewer 
design.  The  difference  between  the  value  of  V  determined  by 
the  omission  of  this  term  and  the  value  of  V  found  by  including 
it  is  less  than  1  per  cent  for  all  slopes  greater  than  1  in  1,000 
for  8  inch  pipe  (#  =  0.167  feet).  As  the  diameter  of  the  pipe  or 
the  hydraulic  radius  of  the  channel  increases  up  to  a  diameter  of 
13.02  feet  (#  =  3.28  feet),  the  difference  becomes  less  and  at  this 
value  of  R  there  is  no  difference  whether  the  slope  is  included  or 
not.  For  larger  pipes  the  difference  increases  slowly.  For  a 
16  foot  pipe  (R  =  4  feet)  on  a  slope  of  1  in  1,000  the  difference  is 
less  than  0.2  per  cent,  and  on  a  slope  of  1  in  10,000  the  difference 
is  approximately  1  per  cent.  Flatter  slopes  than  these  are 


54  THE  HYDRAULICS  OF  SEWERS 

seldom  used  in  sewer  design,  except  for  very  large  sewers  where 
careful  determinations  of  the  hydraulic  slope  are  necessary.  It 
is  therefore  safe  in  sewer  design  to  use  Kutter's  formula  in  the 

002S 

modified  form  shown  below  in  which  the  term    —  ^—  has  been 

o 

omitted. 


n(V72+41.67n) 

Bazin's  formula  is 


in  which  a  and  j3  are  constants  for  different  classes  of  material. 
For  cast-iron  pipe  a  is  0.00007726  and  (3  is  0,00000647.  This 
formula  is  seldom  used  in  sewerage  design- 

Exponential  formulas  have  been  developed  as  the  result  of 
experiments  which  have  demonstrated  that  V  does  not  vary  as 
the  one  half  power  of  R  and  S  but  that  the  relation  should  be 
expressed  as, 

V=CRP&, 

in  which  p  and  q  are  constants  and  C  is  a  factor  dependent  on 
the  character  of  the  material.  The  various  formulas  coming 
under  this  classification  have  been  given  the  names  of  the  experi- 
menters proposing  them.  Examples  of  these  formulas  are: 
Flamant's,  in  English  units,  for  new  cast  iron  pipe,  which  is, 


and  Lampe's  for  the  same  material  which  is, 
F=203.3#694S555. 

These  formulas  are  useful  only  for  the  material  to  which  they 
apply,  but  they  can  be  used  for  conduits  of  any  shape'.  A.  V. 
Saph  and  E.  W.  Schoder  have  shown  l  that  the  general  formula 
for  all  materials  lies  between  the  limits, 


1  Transactions  of  the  American  Society  of  Civil  Engineers,  Vol.  51,  1903, 
p.  11. 


SOLUTION  OF  FORMULAS  55 

Hazen  and  Williams7  formula  is  in  the  form, 


in  which  C  is  a  factor  dependent  on  the  character  of  the  material 
of  the  conduit.  The  values  of  C  as  given  by  Hazen  and  Williams 
are, 

C  CHARACTER  OF  MATERIAL 

95    Steel    pipe    under    future    conditions.     (Riveted 

steel.) 
100     Cast  iron  under  ordinary  future  conditions  and 

brick  sewers  in  good  condition. 
110    New  riveted  steel,  and  cement  pipe. 
120    Smooth  wood  or  masonry  conduits  under  ordinary 

conditions. 
130    Masonry  conduits  after  some  time  and  for  very 

smooth  pipes  such  as  glass,  brass,  lead,  etc., 

when  old,  and  for  new  cast-iron  pipe  under 

ordinary  conditions. 

This  formula  is  of  as  general  application  as  Kutter's  formula  and 
is  easier  of  solution,  but  being  more  recently  in  the  field  and 
because  of  the  ease  of  the  solution  of  Kutter's  formula  by  dia- 
grams it  is  not  in  such  general  use.  Exponential  formulas  are 
used  more  in  waterworks  than  in  sewerage  practice. 
Manning's  formula  is  in  the  form, 


n 

in  which  n  is  the  same  as  for  Kutter's  formula.  Charts  for  the 
solution  of  Manning's  formula  are  given  in  Eng.  News-Record, 
Vol.  85,  1920,  p.  837. 

36.  Solution  of  Formulas.  —  The  solution  of  even  the  simplest 
of  these  formulas,  such  as  Flamant's,  is  laborious  because  of  the 
exponents  involved.  Darcy's  and  Kutter's  formulas  are  even 
more  cumbersome  because  of  the  character  of  the  coefficient. 
The  labor  involved  in  the  solution  of  these  formulas  has  resulted 
in  the  development  of  a  number  of  diagrams  and  other  short  cuts. 
Since  each  formula  involves  three  or  more  variables  it  cannot  be 
represented  by  a  single  straight  line  on  rectangular  coordinate 
paper.  The  simplest  form  of  diagram  for  the  solution  of  three 
or  more  variables  is  the  nomograph,  an  example  of  which  is  shown 


56 


THE  HYDRAULICS  OF  SEWERS 


-48  3 

-g 

36.5 


24 


1.5- 


2  - 


in  Fig.  13  for  the  solution  of  Flamant's  formula.     A  straight-edge 

placed  on  any  two  points  of 
the  scales  of  two  different  ver- 
tical lines  will  cross  the  other 
line  at  a  point  on  the  scale  cor- 
responding to  its  correct  value 
in  the  formula.  Such  a  diagram 
is  in  common  use  for  the 
solution  of  problems  for  the 
flow  of  water  in  cast-iron 
pipe. 

Fig.  14   has   been  prepared 
to     simplify    the     solution    of 
Hazen   and    Williams'    formu- 
la.     The    scales    of    slope  for 
different    classes     of     material 
are    shown    on    vertical    lines 
of    the    slope    line, 
these  scales  must   be 


0.00003 


0.0001 


0.001 


0.01 


O.I 


10- 


FIG.  13.  —  Diagram  for  the  Solution  of 


Flamant's  Formula  for  the  Flow  of  to  tne 

Water  in  Cast-iron  Pipe.  For  use 

projected    horizontally   on   the 

slope  line.     The  scales  for  other  factors  are  shown  on  independent 
reference  lines. 

For  example  let  it  be  required  to  find  the  loss  of  head  in 
a  12  inch  pipe  carrying  1  cubic  foot  per  second  when  the 
coefficient  of  roughness  is  100.  .  A  straight-edge  placed 
at  1.0  cubic  feet  per  second  on  the  quantity  scale,  and  12 
inches  on  the  diameter  scale  crosses  the  slope  line  at  0.00092 
opposite  the  slope  scale  for  c=100.  It  crosses  the  velocity 
line  at  1,31  feet  per  second. 

Kutter's  formula  is  the  most  commonly  used  for  sewer  design 
and  has  been  generally  accepted  as  a  standard  in  spite  of  its 
cumbersomeness.  Fig.  15  is  a  graphical  solution  of  Kutter's 
formula  for  small  pipes,  and  Fig.  16  for  larger  pipes.  The  dia- 
grams are  drawn  on  the  nomographic  principle  and  give  solutions 
for  a  wide  range  of  materials,  but  they  are  specially  prepared  for 
the  solution  of  problems  in  which  n=.015.  In  their  preparation 
the  effect  of  the  slope  on  the  coefficient  has  been  neglected.  Fig. 
17  is  drawn  on  ordinary  rectangular  coordinate  paper  and  can  be 
used  only  for  the  solution  of  problems  in  which  n=.015.  Both 
diagrams  are  given  for  practice  in  the  use  of  the  different  types. 


SOLUTION  OF  FORMULAS 


57 


-  10                                     48" 
-9 

:8                       42" 

I7                                         36" 
-6                                         33" 

30" 
-5 

:                         IT 

r4                                        24" 

21" 
,3                                       20" 

18" 

15" 
-I 

10" 

9" 

:°-9  \                    8" 

Values  of  C 

0     0    §2§S§ 

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0.8- 

0.9- 
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2- 

c  ?  5- 

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FIG.  14.— Diagram  for  the  Solution  of  Hazen  and  Williams'  Formula. 


58 


THE  HYDRAULICS  OF  SEWERS 


Values  of  n 


0 

\ 

S  " 

{4T 

1  " 

x 

^             J  * 

_      £n 

x 

•  JU 

7 

X        *^  ^  J 

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v      ^  c 

i^  s- 

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^^ 

2     c 

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p 

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Q 

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N, 

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1.0  -n 

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6.5  T 

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FIG.  15. — Diagram  for  the  Solution  of  Kutter's  Formula. 

For  values  of  n  between  0.010  and  0.020.     Specially  arranged  for  «  =  0.015.     Values  of 
Q  from  G.I  to  10  second-feet, 


SOLUTION  OF  FORMULAS 


59 


Values  of  n 


>  -  OslhO     LT> 


onn     i^x^x 

^  ?. 

L\J 

X      "^ 

1  9 

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R     5 

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00      .  ^x^ 

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700     S^S^ 

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16' 

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Al 

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15- 

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16- 

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17- 

18- 

•      19- 

.2 

20- 

.3 

.4 

.5 

.6 

I 

\  n 

FIG.  16. — Diagram  for  the  Solution  of  Kutter's  Formula. 

For  values  of  n  between  0.010  and  0.020.     Specially  arranged  for  n  =  0.015. 
Q  from  10  to  1,000  second-feet. 


Values  of 


60 


THE  HYDRAULICS  OF  SEWERS 


060-0 

oeo'o 

OZO'O 
090'0 
OSO'O 


JOO'O 
6000'0 
8000'Q 
ZOOO'O 
9000-Q 
SOOO'O 


USE  OF  DIAGRAMS 


61 


In  Figs.  15  and  16  the  diameter  scales  are  varied  for  different 
values  of  the  roughness  coefficient  n.  The  velocity  scale  is  shown 
only  for  a  value  of  n=.015. 
The  velocity  for  other  values 
of  n  can  be  determined  by  the 
method  given  in  the  following 
paragraphs. 

37.  Use  of  Diagrams.— 
There  are  five  factors  in 
Kutter's  formula:  n,  Q,  V,  d 
(or  R),  and  S.  If  any  three  of 
these  are  given  the  other  two 
can  be  determined,  except  when 
the  three  given  are  Q,  F,  and  d. 
These  three  are  related  in  the 
form  Q=AV,  which  is  inde- 
pendent of  slope  or  the  char- 
acter of  the  material.  There 


Values  of  n. 

FIG.  18. — Conversion  Factors  for 
Kutter's  Formula. 


are  only  nine  different  com- 
binations possible  with  these 
five  factors,  which  will  be  met 

in  the  solution  of  Kutter's  formula.  The  solution  of  the 
problems  by  means  of  the  diagrams  is  simple  when  the  data 
given  include  n=.015.  For  other  given  values  of  n  the  solu- 
tion is  more  complicated.  Results  of  the  solution  of  types  of 
each  of  the  nine  problems  are  given  in  Table  17  and  the 
explanatory  text  below. 

//  n  is  given  and  is  equal  to  .015,  the  solution  is  simple. 

For  example  in  Table  17  case  1,  example  1;  to  be  solved 
on  Fig.  15.  Place  a  straight-edge  at  1.0  on  the  Q  line  and 
at  6  inches  on  the  diameter  line  for  n=  .015.  The  slope 
and  the  velocity  will  be  found  at  the  intersection  of  the 
straight-edge  with  these  respective  scales. 

All  problems  in  which  n  is  given  as  .015  and  the  solution  for  which 
falls  within  the  limits  of  Fig.  15  or  16  should  be  solved  by  placing 
a  straight-edge  on  the  two  known  scales  and  reading  the  two 
unknown  results  at  the  intersection  of  the  straight-edge  and  the 
remaining  scales. 

For  example  in  case  1,  example  2  find  the  intersection 
of  the  horizontal  line  representing  Q=100  with  the  sloping 


62 


THE  HYDRAULICS  OF  SEWERS 


diameter  line  representing  d—4S  inches.  The  vertical 
slope  line  passing  through  this  point  represents  S=  .0065 
and  the  sloping  velocity  line  passing  through  this  point 
represents  8.5  feet  per  second. 

In  general  problems  in  which  n=.015,  can  be  solved  on  Fig.  17 
by  finding  the  intersection  of  the  two  lines  representing  the  given 
data,  and  reading  the  values  of  the  remaining  variables  represented 
by  the  other  two  lines  passing  through  this  point. 

TABLE  17 
SOLUTIONS  OF  PROBLEMS  BY  KUTTER'S  FORMULA 


Ex- 

Givei 

i 

Foun 

d 

Case 

ample 

n 

Q 

V 

d 

S 

n 

Q 

V 

d 

S 

1 
1 

1 
2 

0.015 
.015 

1.0 

100.0 

2.5 

6 

5.0 

8.5 

0.0575 
.0065 

1 

1 
2 

3 

4 
1 

.020 
.020 
015 

1.0 
100.0 
5  0 

6 

48 

0  0003 

5.0 

8.5 
1  2 

28 

.13 
.0125 

2 

2 

010 

5  0 

0003 

1  7 

23  5 

3 
3 

1 
2 

.015 

018 

18 
18 

.002 
0008 

40 
2  0 

2.25 
1  1 

4 

1 

015 

2  0 

?  5 

12 

00475 

4 
5 

2 
1 

.011 
015 

2.0 

2.5 
5  0 

36 

35  0 

12 

.0022 
0038 

6 

1 

018 

5  0 

001 

185  0 

80 

7 
7 
8 
9 

1 

2 
1 
1 

3.0 
50.0 
6.0 

2.5 
4  ? 

18 
36 

66 

.002 
.005 
.003 
00059 

0.019 
.012 
.018 
Oil 

100  0 

1.7 
7.0 

21 

I/  n  is  given  and  is  not  equal  to  .015  the  solution  is  not  so  simple. 
In  Fig.  15  and  16  the  diagram  is  so  drawn  that  the  position  of 
the  diameter  scales  for  all  values  of  n  is  fixed  on  the  vertical 
"  diameter  "  line.  The  scales  of  diameter  change  for  each  value 
of  n.  These  scales  of  diameter  are  shown  for  each  value  of  n 
from  .010  to  .020  on  vertical  lines  to  the  left  of  the  "  diameter  " 
line.  For  use,  the  proper  diameter  scale  for  any  given  value  of  n 
must  be  projected  horizontally  upon  the  vertical  "  diameter " 
line.  The  velocity  can  be  determined  on  Fig.  15  and  16,  only 


USE  OF  DIAGRAMS  63 

when  the  diameter  has  been  determined  and  then  only  when  the 
diameter  scale  for  n  equal  .015  is  used,  since  the  only  scale  shown 
for  velocity  is  for  n=  .015. 

For  example,  in  Case  1,  Example  3  there  are  given 
n=  .020,  Q,  and  d.  Find  the  intersection  of  the  vertical 
line  for  n  =  .  020  with  the  sloping  diameter  line  for  d  =  6 
inches.  Project  the  intersection  horizontally  to  the  right 
to  the  vertical  "diameter"  line.  Place  a  straight-edge  at 
this  point  and  at  Q  =  1.0  on  the  quantity  scale.  The 
required  value  of  S  is  read  at  the  intersection  of  the  straight- 
edge and  the  slope  scale  and  is  equal  to  0.13.  The  inter- 
section of  the  straight-edge  in  this  position  with  the  velocity 
scale  is  not  the  required  value  of  the  velocity  since  the 
velocity  scale  is  made  out  for  n=  .015  and  not  .020.  It  is 
necessary  to  change  the  position  of  the  straight-edge  so  that 
it  may  lie  on  Q  equal  1.0  and  on  d  equal  6  inches  for  n 
equal  .015.  The  value  of  V  is  shown  in  this  position  as  5 
feet  per  second. 

The  reverse  process  for  Fig.  15  and  16  is  illustrated 
by  Case  4,  Example  2  in  which  n=  .011  and  Q  and  V  are  also 
given.  When  Q  and  V  are  given  the  value  of  d  is  fixed 
independent  of  all  other  factors.  Therefore  the  value  of  d 
can  be  read  from  the  scale  with  n=  .015  and  is  found  to  be 
12  inches.  Now  find  the  value  of  d=  12  inches  on  the  scale 
for  n=  .011  and  project  on  to  the  "diameter"  line.  Place 
the  straight-edge  at  this  point  and  at  Q  =  2.  The  required 
slope  is  read  as  .  0022. 

Fig.  17  is  prepared  for  the  solution  of  problems  in  which 
n=.015  only.  For  problems  in  which  n  has  some  other  value  it 
is  necessary  to  transform  the  data  to  equivalent  conditions  in 
which  n=.015.  This  is  done  by  means  of  the  conversion  factors 
shown  in  Fig.  18.  The  given  slope  or  velocity  is  multiplied  by 
the  proper  factor  to  convert  from  or  to  the  value  of  n=  .015, 

For  example  in  Case  1,  Example  4  there  are  given 
n=  .  020,  Q,  and  d.  With  Q  and  d  given  the  value  of  V  can 
be  read  from  Fig.  17  without  conversion.  The  correspond- 
ing value  of  S  for  n=  .015  is  .0065.  It  is  now  necessary  to 
use  the  transformation  diagram  Fig.  18.  The  hydraulic 
radius  of  the  given  pipe  is  one  foot.  On  Fig.  18  at  the  inter- 
section of  the  slope  line  for  #=1.0  foot  and  n  =  .  020  the 
value  of  the  factor  is  read  as  1.92.  Since  the  given  n  is 
for  rougher  material  than  that  represented  by  n=  .015  the 
required  slope  must  be  greater  than  forn=  .015  to  give  the 


64  THE  HYDRAULICS  OF  SEWERS 

same  velocity.  It  is  therefore  necessary  to  multiply 
.  0065  X 1 . 92  and  the  required  slope  is  .0125. 

In  Case  6,  Example  1  there  are  given  n=  .018,  d,  and  S, 
The  remaining  factors  are  to  be  solved  by  Fig.  17.  Solve 
first  as  though  n=  .015  in  order  to  find  an  approximate 
value  of  d  or  R.  In  this  case  it  is  evident  that  d  is  greater 
than  57  inches.  The  value  of  R  is  therefore  about  1.25. 
Referring  to  Fig.  18  the  conversion  factor  for  the  slope  for 
n=  .018  is  about  1.52.  Since  the  given  slope  for  n=  .018 
is  .001,  for  an  equal  velocity  and  for  n=  .015  the  slope 
should  be  less.  Therefore  in  reading  Fig.  17  it  is  necessary 

001 

to  use  a  slope  of  ^— =^  =  .00066.     The  diameter  is  found  to 
1 .  oZ 

be  about  80  inches.  Since  this  is  nearer  to  the  correct 
diameter  the  value  of  the  conversion  factor  must  be  cor- 
rected for  this  approximation.  The  hydraulic  radius  for  an 
80  inch  pipe  is  1.67  feet,  and  the  conversion  factor  from 
Fig.  18  is  about  1.48.  The  slope  for  ft  =.015  should  be 

001 
therefore  j-^=.  000675  and  from  Fig.   17  the  required 

diameter  and  quantity  are  read  as  80  inches  and  185  second 
feet,  respectively, 

//  n  is  not  given  but  must  be  solved  for,  the  solution  on  Fig. 
15  and  16  is  relatively  simple.  The  desired  value  of  n  is  read  at 
the  intersection  of  the  sloping  diameter  line  representing  the 
known  diameter  and  the  horizontal  projection  of  the  intersection 
of  the  straight-edge  with  the  vertical  "  diameter  "  line. 

For  example  in  Case  7,  Example  1  there  are  given  Q, 
d,  and  S.  Lay  the  straight-edge  on  the  given  values  of 
Q  =  3  and  S=  .002.  At  the  point  where  the  straight-edge 
crosses  the  vertical  "diameter"  line  project  a  horizontal 
line  to  the  sloping  diameter  line  for  d=18  inches.  The 
vertical  line  passing  through  this  point  represents  a  value  of 
n=  .019.  In  order  to  find  the  value  of  V  lay  the  straight- 
edge on  Q  =  3  and  d=  18  inches  for  n=  .015.  The  value  of 
V  is  read  as  1.7. 

A  slightly  different  condition  is  illustrated  in  the  solu- 
tion of  Case  8,  Example  1  in  which  Q,  V  and  S  are  given. 
Determine  first  the  value  of  d  as  though  n=  .015.  Then 
proceed  to  determine  n  as  in  the  preceding  examples. 

The  solution  for  an  unknown  value  of  n  on  Fig.  17  is  not  so 
simple.  It  must  be  determined  by  working  backwards  from  the 
conversion  factor. 


FLOW  IN  CIRCULAR  PIPES  PARTLY  FULL  65 

For  example  in  Case  7,  Example  2  there  are  given  Q, 
d,  and  S.  The  value  of  V  is  read  directly  as  though  n  =  .015 
as  7  feet  per  second.  The  value  of  S  read  for  n=  .015  is 
is  .0075.  But  the  given  slope  is  .005.  Since  the  given 
slope  is  flatter  than  that  for  n=  .015  the  conversion  factor 

005 
is  less  than  unity  and  is  therefore  -^yr=  =  0  .  67.     With  this 


value  of  the  conversion  factor  and  the  value  of  R  given  as 
0.75  the  value  of  n  is  read  from  Fig.  18  as  slightly  greater 
than  .012. 

38.  Flow  in  Circular  Pipes  Partly  Full.  —  The  preceding 
examples  have  involved  the  flow  in  circular  pipes  completely 
filled.  The  same  methods  of  solution  can  be  used  for  pipes 
flowing  partly  full  except  that  the  hydraulic  radius  of  the  wetted 
section  is  used  instead  of  the  diameter  of  the  pipe.  Diagrams 
are  used  to  save  labor  in  finding  the  hydraulic  radius  and  the 
other  hydraulic  elements  of  conduits  flowing  partly  full. 

The  hydraulic  elements  of  a  conduit  for  any  depth  of  flow  are  : 
(a)  The  hydraulic  radius,  (6)  the  area,  (c)  the  velocity  of  flow, 
and  (d)  the  quantity  or  rate  of  discharge.  The  velocity  and 
quantity  when  partly  full  as  expressed  in  terms  of  the  velocity 
and  quantity  when  full  as  calculated  by  Kutter's  formula  will 
vary  slightly  with  different  diameters,  slopes  and  coefficients  of 
roughness.  The  other  elements  are  constant  for  all  conditions 
for  the  same  type  of  cross-section.  The  hydraulic  elements  for 
all  depths  of  a  circular  section  for  two  different  diameters  and 
slopes  are  shown  in  Fig.  19.  The  differences  between  the 
velocity  and  quantity  under  the  different  conditions  are  shown 
to  be  slight,  and  in  practice  allowance  is  seldom  made  for  this 
discrepancy. 

In  the  solution  of  a  problem  involving  part  full  flow  in  a  cir- 
cular conduit  the  method  followed  is  to  solve  the  problem  as 
though  it  were  for  full  flow  conditions  and  then  to  convert  to 
partial  flow  conditions  by  means  of  Fig.  19,  or  to  convert  from 
partial  flow  conditions  to  full  flow  conditions  and  solve  as  in  the 
preceding  section. 

For  example  let  it  be  required  to  determine  the  quantity 
of  flow  in  a  12-inch  diameter  pipe  with  n=  .015  when  on  a 
slope  of  .  005  and  the  depth  of  flow  is  3  inches.  First  find 
the  quantity  for  full  flow.  From  Fig.  15  this  is  2.0  cubic 
feet  per  second.  The  depth  of  flow  of  3  inches  is  one-fourth 


66 


THE  HYDRAULICS  OF  SEWERS 


or  0.25  of  the  full  depth  of  12  inches.  From  Fig.  19,  run- 
ning horizontally  on  the  0.25  depth  line  to  meet  the  quantity 
curve,  the  proportionate  quantity  at  this  depth  is  found  to 
be  on  the  0.13  vertical  line,  and  the  quantity  of  flow  is 
therefore  2  X0.13  =  0.26  cubic  feet  per  second. 


0        O.t      0.2     0.3      0.4      0.5      0.6      0.7      0.8     0.9       1.0      1.1      1.2 
Hydraulic   Elements  in  Terms  of  Hydraulic  Elements  for  Full  Section. 

FIG.  19. — Hydraulic  Elements  of  Circular  Sections. 

s  =  .0004 


d  =  12'  0" 
d=    I'O" 


n  =  .015 
n  =  .013 


Another  problem,  involving  the  reversal  of  this  process  is 
illustrated  by  the  following  example: 

Let  it  be  required  to  determine  the  diameter  and  full 
capacity  of  a  vitrified  pipe  sewer  on  a  grade  of  0.002  if  the 
velocity  of  flow  is  3.0  feet  per  second  when  the  sewer  is 
discharging  at  30  per  cent  of  its  full  capacity,  the  depth  of 
flow  being  12  inches.  From  Fig.  19  the  depth  of  flow  when 
the  sewer  is  carrying  30  per  cent  of  its  full  capacity  is  0 . 38 
of  its  full  depth.  Since  the  partial  depth  is  12  inches 

12 

the  full  diameter  is  =31.6  inches.     The  velocity  of 

U .  oo 

flow  at  38  per  cent  depth  is  86  per  cent  of  the  full  velocity. 
Since  the  velocity  given  is  3.0  feet  per  second,  the  full 

3  0 

velocity  is  -^  =  3.5  feet  per  second.  With  a  full  ve- 
locity of  3.5  feet  per  second  and  a  diameter  of  31.6  inches 
from  Fig.  16  the  full  capacity  of  the  sewer  is  18  cubic  feet 
per  second. 


SECTIONS  OTHER  THAN  CIRCULAR  67 

39.  Sections  Other  than  Circular. — The  ordinary  shape  used 
for  small  sewers  is  circular.  The  difficulty  of  constructing  large 
sewers  in  a  circular  shape,  special  conditions  of  construction  such 
as  small  head  room,  soft  foundations,  etc.,  or  widely  fluctuating 
conditions  of  flow  have  led  to  the  development  of  other  shapes. 
For  conduits  flowing  full  at  all  times  a  circular  section  will  carry 
more  water  with  the  same  loss  of  head  than  any  other  section 
under  the  same  conditions.  In  any  section  the  smaller  the  flow 
the  slower  the  velocity,  an  undesirable  condition.  The  ideal 
section  for  fluctuating  flows  would  be  one  that  would  give  the 
same  velocity  of  flow  for  all  quantities.  Such  a  section  is  yet  to 
be  developed.  Sections  have  been  developed  that  will  give  rela- 
tively higher  velocities  for  small  quantities  of  flow  than  are  given 
by  a  circular  section.  The  best  known  of  these  sections  is  the 
egg  shape,  the  proportions  and  hydraulic  elements  of  which  are 
shown  in  Fig.  20.  Other  shapes  that  have  the  same  property, 
but  which  were  not  developed  for  the  same  purpose  are  the  rect- 
angular, the  U-shape,  and  the  section  with  a  cunette.  The  egg- 
shaped  section  has  been  more  widely  used  than  any  other  special 
section.  It  is,  however,  more  difficult  and  expensive  to  build 
under  certain  conditions,  and  has  a  smaller  capacity  when  full 
than  a  circular  sewer  of  the  same  area  of  cross-section.  Various 
sections  are  illustrated  in  Fig.  22  and  23. 

The  U-shaped  section  is  suitable  where  the  cover  is  small,  or 
close  under  obstructions  where  a  flat  top  is  desirable  and  the 
fluctuations  of  flow  are  so  great  as  to  make  advantageous  a  special 
shape  to  increase  the  velocity  of  low  flows.  The  proportions  of  a 
U-shaped  section  are  shown  in  Fig.  23  (6).  Other  sections  used 
for  the  same  purpose  are  the  semicircular  and  special  forms  of 
the  rectangular  section. 

The  proportions  and  the  hydraulic  elements  of  the  square- 
shaped  section  are  shown  in  Fig.  21.  This  is  useful  under  low 
heads  where  a  flat  roof  is  required  to  carry  heavy  loads,  and  the 
fluctuations  of  flow  are  not  large. 

Sections  with  cunettes  have  not  been  standardized.  A  cunette 
is  a  small  channel  in  the  bottom  of  a  sewer  to  concentrate  the  low 
flows,  as  shown  in  Fig.  22  (7).  A  cunette  can  be  used  in  any 
shape  of  sewer. 

Sections  developed  mainly  because  of  the  greater  ease  of  con- 
struction under  certain  conditions  are  the  basket  handle,  the  gothic, 


THE  HYDRAULICS  OF  SEWERS 


0        O.I       0.2      0.3     0.4      0.5     0.6     0.7      0.8       0.9      1.0       I.I        1.2 
Hyoraulic   Elements  in  Terms  of  Hydraulic  Elements  for  Full  Section 

FIG.  20. — Hydraulic  Elements  of  an  Egg-shaped  Section. 

d  =  6'0"  s  =  .00065  n  =  .015 


0        0.1       0.2     0.3      0.4     0.5     0.6     0.7      0.8     0.9       1.0      1.1 
Hydraulic  Elements  in  Terms  of  Hydraulic  Elements  for  Full  Section. 

FIG.  21. — Hydraulic  Elements  of  a  Square  Section. 

d  =  10'  0"  s  =  .0004  n  =  .015 


SECTIONS  OTHER  THAN  CIRCULAR 


the  catenary,  and  the  horse  shoe.  Some  of  these  shapes  are  shown 
in  Fig.  22  and  23.  They  are  suitable  for  large  sewers  on  soft 
foundations,  where  it  is  desirable  to  build  the  sewer  in  three 
portions,  such  as,  invert,  side  walls,  and  arch.  They  are  also 
suitable  for  construction  in  tunnels  where  the  shape  of  the  sewer 
conforms  to  the  shape  of  the  timbering,  or  in  open  cut  work  where 
the  shape  of  the  forms  are  easier  to  support. 

Problems  of  flow  in  all  sections  can  be  solved  by  determining 
the  hydraulic  radius  involved,  and  substituting  directly  in  the 
desired  formula,  or  by  the  use  of  one  of  the  diagrams  after  con- 
verting to  the  equivalent  circular  diameter.  The  determina- 
tion of  the  hydraulic  radius  of  these  special  sections  is  laborious, 
and  hence  other  less  difficult  methods  are  followed.  Problems 
are  more  commonly  solved  by  converting  the  given  data  into  an 
equivalent  circular  sewer,  solving  for  the  elements  of  this  cir- 
cular sewer  and  then  reconverting  into  the  original  terms,  or  by 
working  in  the  other  direction.  The  hydraulic  elements  of  vari- 
ous sections  when  full  are  given  in  Table  18. 


TABLE  18 
HYDRAULIC  ELEMENTS  OF  SEWER  SECTIONS. 


SEWERS  FLOWING  FULL 


Vert. 

Area 

Dia.  D 

in 

Hy- 

in  Terms 

draulic 

Section 

Terms 
Vertical 
Diameter 
Squared 

Radius  in 
terms  of 
Vertical 

Din     D 

of 
Dia.  d 
of  Equiv- 
alent 

Source 

L>2 

.L/  It*  .    i  '  . 

Circular 

Section 

Circular 

0  .  7854 

0.250 

1.000 

Egg  

0.5150 

.  1931 

1.295 

Eng.  Record,  Vol.  72:  608 

Ovoid  

0  .  5650 

.2070 

1.208 

Eng.  Record,  Vol.  72:  608 

Semi-elliptical 

0.8176 

.2487 

1.041 

Eng.  News,  Vol.  71:  552 

Catenary  

0  .  6625 

.2237 

1.1175 

Eng.  Record,  Vol.  72:  608 

Horseshoe  

0.8472 

.2536 

0.985 

Eng.  Record,  Vol.  72:  608 

Basket  handle.   . 

0.8313 

.2553 

0.979 

Eng.  Record,  Vol.  72.  608 

Rectangular 

1.3125 

.2865 

.7968 

Hydraulic  D^ms.  and  Tbls. 

Garrett 

Square  (3  sides  wet)  . 

1.0000 

.333 

.7500 

Eng.  Record,  Vol.  72:  608 

Square  (4  sides  wet)  . 

1.0000 

.250 

1.0000 

Eng  Record,  Vol.  72:  608 

1 

70 


THE  HYDRAULICS  OF  SEWERS 


7*- 


— V_ 


/-*_[_  _r- — ^  ......      ^^^.^ 

U    <- 20'- 


1.  Standard  Egg-shaped  Section,  North     2.  Rectangular  Section,  Omaha,  Nebraska, 
Shore  Intercepter,  Chicago,  Illinois.  Eng.  Contracting,  Vol.  46,  p.  49. 


3.  Trench  in  firm  4.  Trench  in 

ground.  Rock. 

NOTE — Underdrains  and  Wedges  to  be 
used  only  when  Ordered  by  the 
Engineer. 


^  fact  Brick 


7.  Brick  and  Concrete  Sewer  showing 
cunette. 


Concrete 


6 'ravel 'or  r/    j   i 

Broken  Stone.-'''    Plank ,     Filling 

}<— 13-10"- 

5.  Soft  Founda-  6.  Wet  8.  Brick  and  Concrete  Sewer,  Evanston, 

tion.  ground.  111.,  Eng.  Contracting,  Vol.  46,  p.  227. 

FIG.  22. 


SECTIONS  OTHER  THAN  CIRCULAR 


71 


flLjJM- 

!       *t1 

i            i       U 

'.rt                 '.           '.  <o 

?-,AEf^-t-i-^:/— 

& 

6"- 

^rr+^-~ 

1.  Tunnel  Sections.  2.  Open  Cut  Sections. 

Type  A.         Type  B.  Type  C.          Type  D. 

Where  Rock  is  Where  Rock  is  Where  Rock  is  Where  Rock      16'  6"  Sewer.     Where  Rock 
more  than  16'  more    than   7'        between       drops  below       25'  Fill  is  above 

above  Spring-  an     less  than  Springing  Line  Springing  Line  Springing 

ing  Line.  16'  above      and   7'    above  on  either  Side.  Line 

Springing  Line  Springing  Line 
on  both  Sides,  on  both  Sides. 
Mill  Creek  Sewer,  St.  Louis,  Eng.  Record,  Vol.  70,  pp.  434,  435. 


Re,£+t+Z~ 
*„•$«' 


Soft  Hard 

Ground.  Ground. 

3.  Circular  Concrete  Section  in  Soft  and  Hard     4.  Semi-Elliptical  Section,  Louisville,  Ky. 
Ground,  Eng.  Record,  Vol.  59,  p.  570,  Eng.  News,  Vol.  62,  p.  416. 


In  Soft  Ground.  In  Rock. 

5.  Reinforced  Concrete  Sewer,  Harlem  Creek, 
St.  Louis,  Eng.  News,  Vol.  60,  p.  131. 

FIG.  23. 


6.  U-Shaped  Section,  San  Francisco, 
Eng.  News,  Vol.  73,  p.  310. 


72  THE  HYDRAULICS  OF  SEWERS 

Equivalent  sections  are  sections  of  the  same  capacity  for  the 
same  slope  and  coefficient  of  roughness.  They  have  not  neces- 
sarily the  same  dimensions,  shape,  nor  area.  The  diameter  of 
the  equivalent  circular  section  in  terms  of  the  diameter  of  each 
special  section  shown  is  given  in  Table  18.  The  inside  height  of 
a  sewer  is  spoken  of  as  its  diameter. 

For  example  let  it  be  required  to  determine  the  rate  of 
flow  in  a  54-inch  egg-shaped  sewer  on  a  slope  of  0  .  001  when 
n=  .015.  First  convert  to  the  equivalent  circle.  From 

Table  18  the  diameter  of  the  equivalent  circle  is  times 


the  diameter  of  the  egg-shaped  sewer,  which  becomes  in 
this  case  43  inches.  From  Fig.  16  the  capacity  of  a  circular 
sewer  of  this  diameter  with  S  =  0.001  and  n=  .015  is  28 
cubic  feet  per  second,  which  by  definition  is  the  flow  in  the 
egg-shaped  sewer. 

As  an  example  of  the  reverse  process  let  it  be  required 
to  find  the  velocity  of  flow  in  an  egg-shaped  sewer  flowing 
full  and  equivalent  to  a  48-ineh  circular  sewer.  Both  sewers 
are  on  a  slope  of  0  .  005  and  have  a  roughness  coefficient  of 
n=  .015.  It  is  first  necessary  to  find  the  quantity  of  flow 
in  the  circular  sewer,  which  by  definition  is  the  quantity  of 
flow  in  the  equivalent  egg-shaped  sewer.  The  velocity  of 
flow  in  the  egg-shaped  sewer  is  found  by  dividing  this 
quantity  by  the  area  of  the  egg-shaped  section.  As  read 
from  the  diagram  the  quanity  of  flow  is  90  cubic  feet  per 
second.  From  Table  18  the  area  of  the  egg-shaped  sewer  is 
0  .  5  ID2  where  D  is  the  diameter  of  the  egg-shaped  sewer,  and 
D  =  1  .  295d  where  d  is  the  diameter  of  the  equivalent  cir- 
cular sewer.  Therefore  the  area  equals  (0  .  51)  X  (1  .295  X4)2 

90 

=  13.5  square  feet  and  the  velocity  of  flow  is  =6.7 

lo  .  o 

feet  per  second.  This  is  slightly  less  than  the  velocity 
in  the  circular  section. 

Some  lines  for  egg-shaped  sewers  have  been  shown  on  Fig.  17 
by  which  solutions  can  be  made  directly.  For  other  shapes,  and 
for  sizes  of  egg-shaped  sewers  not  found  on  Fig.  17  the  preceding 
method  or  the  original  formula  must  be  used  for  solution.  Prob- 
lems in  partial  flow  in  special  sections  are  solved  similarly  to 
partial  flow  in  circular  sections,  by  converting  first  to  the  condi- 
tions of  full  flow  or  by  working  in  the  opposite  direction. 

40.  Non-uniform  Flow.  —  In  the  preceding  articles  it  is  assumed 
that  the  mean  velocity  and  the  rate  of  flow  past  all  sections  are 


NON-UNIFORM   FLOW  73 

constant.  This  condition  is  known  as  steady,  uniform  flow.  In 
this  article  it  will  be  assumed  that  conditions  of  steady  non- 
uniform  flow  exist,  that  is,  the  rate  of  flow  past  all  sections  is 
constant,  but  the  velocity  of  flow  past  these  sections  is  different 
for  different  sections.  Under  such  conditions  the  surface  of  the 
stream  is  not  parallel  to  the  invert  of  the  channel.  If  the  velocity 
of  flow  is  increasing  down  stream  the  surface  curve  is  known  as 
the  drop-down  curve.  If  the  velocity  of  flow  is  decreasing  down 
stream  the  surface  curve  is  known  as  the  backwater  curve.  The 
hydraulic  jump  represents  a  condition  of  non-uniform  flow  in 
which  the  velocity  of  flow  decreases  down  stream  in  such  a  manner 
that  the  surface  of  the  stream  stands  normal  to  the  invert  of  the 
channel  at  the  point  where  the  change  in  velocity  occurs.  Above 
and  below  this  point  conditions  of  uniform  flow  may  exist. 

Conditions  of  non-uniform  flow  exist  at  the  outlet  of  all  sewers, 
except  under  the  unusual  conditions  where  the  depth  of  flow  in 
the  sewer  under  conditions  of  steady,  uniform  flow  with  the  given 
rate  of  discharge  would  raise  the  surface  of  water  in  the  sewer,  at 
the  point  of  discharge,  to  the  same  elevation  as  the  surface  of  the 
body  of  water  into  which  discharge  is  taking  place.  By  an  appli- 
cation of  the  principles  of  non-uniform  flow  to  the  design  of  out- 
fall sewers,  smaller  sewers,  steeper  grades,  greater  depth  of  cover, 
and  other  advantages  can  be  obtained. 

The  backwater  curve  is  caused  by  an  obstruction  in  the  sewer, 
by  a  flattening  of  the  slope  of  the  invert,  or  by  allowing  the  sewer 
to  discharge  into  a  body  of  water  whose  surface  elevation  would 
be  above  the  surface  of  the  water  in  the  sewer,  at  the  point  of 
discharge,  under  conditions  of  steady,  uniform  flow  with  the  given 
rate  of  discharge. 

The  drop-down  curve  is  caused  by  a  sudden  steepening  of  the 
slope  of  the  invert;  by  allowing  a  free  discharge;  or  by  allowing  a 
discharge  into  a  body  of  water  whose  surface  elevation  would  be 
below  the  surface  of  the  water  in  the  sewer,  at  the  point  of  dis- 
charge, under  conditions  of  steady,  uniform  flow  with  the  given 
rate  of  discharge.  The  last  described  condition  is  common  at 
the  outlet  of  many  sewers,  hence  the  common  occurrence  of  the 
drop-down  curve. 

The  hydraulic  jump  is  a  phenomenon  which  is  seldom  consid- 
ered in  sewer  design.  If  not  guarded  against  it  may  cause  trouble 
at  overflow  weirs  and  at  other  control  devices,  in  grit  chambers, 


74  THE  HYDRAULICS  OF  SEWERS 

and  at  unexpected  places.  The  causes  of  the  hydraulic  jump 
are  sufficiently  well  understood  to  permit  designs  that  will  avoid 
its  occurrence,  but  if  it  is  allowed  to  occur  the  exact  place  of  the 
occurrence  of  the  jump  and  its  height  are  difficult,  if  not  impos- 
sible, to  determine  under  the  present  state  of  knowledge  con- 
cerning them.  The  hydraulic  jump  will  occur  when  a  high  veloc- 
ity of  flow  is  interrupted  by  an  obstruction  in  the  channel,  by  a 
change  in  grade  of  the  invert,  or  the  approach  of  the  velocity  to 
the  "  critical "  velocity.  The  "  critical "  velocity  is  equal  to 
V0/i,  where  h  is  the  depth  of  flow  and  g  is  the  acceleration  due  to 
gravity.  The  velocity  in  the  channel  above  the  jump  must  be 
greater  than  ^/~gh[,  where  hi  is  the  depth  of  flow  in  the  channel 
above  the  jump.  The  velocity  in  the  channel  below  the  jump 
must  be  greater  than  Vgfo,  where  h%  is  the  depth  of  flow  below 
the  jump.  The  jump  will  not  take  place  unless  the  slope  of  the 

invert  of  the  channel  is  greater  than  ^,  in  which  C  is  the  coeffi- 
cient in  the  Chezy  formula.  With  this  information  it  is  possible 
to  avoid  the  jump  by  slowing  down  the  velocity  by  the  installation 
of  drop  manholes,  flight  sewers,  or  by  other  expedients. 

The  shape  of  the  'drop-down  curve  can  be  expressed,  in  some 
cases,  by  mathematical  formulas  of  more  or  less  simplicity, 
dependent  on  the  shape  of  the  conduit.  The  formula  for  a  circu- 
lar conduit  is  complicated.  Due  to  the  assumptions  which  must  be 
made  in  the  deduction  of  these  formulas,  the  results  obtained  by 
their  use  are  of  no  greater  value  than  those  obtained  by  approxi- 
mate methods.  A  method  for  the  determination  of  the  drop- 
down curve  is  given  by  C.  D.  Hill.1  In  this  method  it  is  necessary 
that  the  rate  of  flow  past  all  sections  shall  be  the  same;  that  the 
depth  of  submergence  at  the  outlet  shall  be  known;  and  that  the 
depth  of  flow  at  some  unknown  distance  up  the  stream  shall  be 
assumed.  The  shape  and  material  of  construction  of  the  sewer 
and  the  slope  of  the  invert  should  also  be  known.  The  problem  is 
then  to  determine  the  distance  between  cross-sections,  one  where 
the  depth  of  flow  is  known,  and  the  other  where  the  depth  of  flow 
has  been  assumed.  This  distance  can  be  expressed  as  follows: 

T_(d2-di)-(Hi-H2)_d'-H' 

S-Si  S'    ' 

1  Municipal  and  County  Engineering,  Vol.  58, 1920,  p.  164. 


NON-UNIFORM   FLOW 


75 


in  which  L  =  the  distance  between  cross-sections; 

di  =  ihe  depth  of  flow  at  the  lower  section; 
e?2  =  the  depth  of  flow  at  the  upper  section; 
HI  =  the  velocity  head  at  the  lower  section; 
//2  =  the  velocity  head  at  the  upper  section; 
$  =  the  hydraulic  slope  of  the  stream  surface; 
Si  =  the  slope  of  the  invert  of  the  sewer. 

In  order  to  solve  such  problems  with  a  satisfactory  degree  of 
accuracy  the  difference  between  di  and  di  should  be  taken  suffi- 
ciently small  to  divide  the  entire  length  of  the  sewer  to  be  investi- 
gated into  a  large  number  of  sections.  The  solution  of  the  prob- 
lem requires  the  determination  of  the  wetted  area,  the  hydraulic 
radius,  and  other  hydraulic  elements  at  many  sections.  The 
labor  involved  can  be  simplified  by  the  use  of  diagrams,  such  as 
Fig.  19,  or  by  specially  prepared  diagrams  such  as  those  accom- 
panying the  original  article  by  C.  D.  Hill.  The  solution  of  the 
problem  can  be  simplified  by  tabulating  the  computations  as 
follows: 

DROP-DOWN  CURVE  COMPUTATION  SHEET 

Uniform  discharge.      Varying  depth 


D.             Q,      A.       v,  *                Sl,                   L_*=». 

A                                                                     &i 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

Depth 

R 

H 

Hi 

di-Hi 

V 

S 

Si 

L 

Elevation 

D 

d 

di 

Sewer 

W.  L. 

At  the  head  of  the  computation  sheet  should  be  recorded  the 
diameter  of  the  sewer  in  feet,  the  assumed  volume  of  flow,  the 
area  of  the  full  cross-section  of  the  sewer,  the  velocity  of  the 
assumed  volume  flowing  through  the  full  bore  of  the  sewer,  and 
the  gradient  or  slope  of  the  invert.  In  the  1st  column  enter  the 


76  THE  HYDRAULICS  OF  SEWERS 

assumed  depth  in  decimal  parts  of  the  diameter  for  each  cross- 
section;  in  the  2nd  column  enter  the  same  depth  in  feet;  in  the 
3rd  column  enter  the  difference  in  feet  between  the  successive 
cross-sections;  in  the  4th  column  enter  the  hydraulic  radius 
corresponding  to  the  depth  at  each  cross-section;  in  the  8th  column 
enter  the  velocity,  equal  to  the  volume  divided  by  the  wetted 
area,  for  each  cross-section;  in  the  5th  column  enter  the  cor- 
responding velocity  head;  in  the  6th  column  enter  the  difference 
between  the  velocity  heads  at  successive  cross-sections;  in  the 
7th  column  enter  the  difference  between  the  quantities  in  the 
third  and  in  the  sixth  columns;  in  the  9th  column  enter  the  hydrau- 
lic slope  corresponding  to  the  velocity  and  hydraulic  radius  of 
each  cross-section;  in  the  10th  column  enter  the  difference  between 
the  hydraulic  slope  and  the  slope  or  gradient  of  the  sewer;  in  the 
llth  column  enter  the  computed  distance  between  successive 
cross-sections;  in  the  12th  column  enter  the  elevation  of  the 
bottom  of  the  sewer  at  each  cross-section;  and  in  the  13th  column 
enter  the  corresponding  elevation  of  the  surface  of  the  water. 

The  table  should  be  filled  in  until  the  distance  to  the  required 
section  is  determined,  or  if  the  distance  is  known,  it  should  be 
filled  in  until  the  depth  of  flow  with  the  assumed  rate  of  discharge 
has  been  checked. 

If  only  the  depth  of  flow  at  some  section  is  known  and  it  is 
required  to  know  the  maximum  rate  of  flow  with  a  free  discharge, 
or  a  discharge  with  a  submergence  at  the  outlet  less  than  the 
depth  of  flow  with  the  maximum  rate  of  discharge,  it  is  necessary 
to  make  a  preliminary  estimate  of  the  maximum  rate  of  flow  in 
order  to  fill  in  the  quantity  Q  at  the  head  of  the  table.  The 
procedure  should  be  as  follows; 

1st.  Assume  a  depth  of  flow  at  the  outlet. 

2nd.  Compute  the  area  (A)  and  the  hydraulic  radius  (R) 
at  the  known  section  and  at  the  outlet. 

3rd.  Determine  the  area  and  the  hydraulic  radius  half  way 
between  these  two  sections  as  the  mean  of  the  areas 
and  the  hydraulic  radii  of  the  two  sections. 

4th.  Determine  the  rate  of  flow  through  the  sewer  from  the 
condition  that  the  difference  in  head  at  the  two 
sections  is  the  head  lost  due  to  friction  caused  by 
the  average  velocity  of  flow  between  the  sections 

(/y2\  / 

equals  ^D  )  Plus  the  £am  m  velocity  head  (  equals 


NON-UNIFORM   FLOW  77 

y  2_  y,2\ 

— —i,  which  when  combined  and  transposed 

result  in  the  expression : 


-A2)  (A2C2R) 

in  which    Q  =rate  of  flow; 

A  =the  area  determined  in  the  3rd  step; 

A  i  =the  area  at  the  upper  cross-section; 

A2  =  the  area  at  the  lower  cross-section; 

C  =the  coefficient  in  the  Chezy  formula; 

g    =the  acceleration  due  to  gravity; 

h    =the  difference  in  elevation  of  the  surface  of  the 

stream  at  the  two  cross-sections; 
I     =  the  distance  between  the  cross-sections; 
R  =  the  hydraulic  radius  determined  in  the  third  step. 
5th.  Continue  this  process  by  assuming  different  depths  at 
the  outlet  until  the  maximum  rate  of  discharge  has 
been  found  by  trial. 

With  this  rate  of  discharge  and  depth  of  flow  at  the  outlet,  the 
depth  of  flow  at  the  known  section  can  be  checked.  If  appreci- 
ably in  error  a  correction  should  be  made  by  the  assumption  of 
a  ditferent  depth  of  flow  at  the  outlet.  The  approximate  character 
of  the  method  is  scarcely  worthy  of  the  refinement  in  the  results 
which  will  be  obtained  by  checking  back  for  the  depth  of  flow  at 
the  known  section.  It  will  be  sufficiently  accurate  to  assume 
the  rate  of  flow  obtained  by  trial  from  the  preceding  expression, 
as  the  maximum  rate  of  discharge  from  the  sewer. 


CHAPTER  V 
DESIGN  OF  SEWERAGE  SYSTEMS 

41.  The  Plan. — Good  practice  demands  that  a  compre- 
hensive plan  for  a  sewerage  system  be  provided  for  the  needs  of  a 
community  for  the  entire  extent  of  its  probable  future-  growth, 
and  that  sewers  be  constructed  as  needed  in  accordance  with 
this  plan. 

Sewerage  systems  may  be  laid  out  on  any  one  of  three  systems : 
separate,  storm,  or  combined.  A  separate  system  of  sewers  is 
one  in  which  only  sanitary  sewage  or  industrial  wastes  or  both 
are  allowed  to  flow.  Storm  sewers  carry  only  surface  drainage, 
exclusive  of  sanitary  sewage.  Combined  sewers  carry  both 
sanitary  and  storm  sewage.  The  use  of  a  combined  or  a  separate 
system  of  sewerage  is  a  question  of  expediency.  Portions  of  the 
same  system  may  be  either  separate,  combined,  or  storm  sewers. 

Some  conditions  favorable  to  the  adoption  of  the  separate 
system  are  where: 

a.  The  sanitary  sewage  must  be  concentrated  at  one 
outlet,  such  as  at  a  treatment  plant,  and  other  outlets 
are  available  for  the  storm  drainage. 

6.  The  topography  is  flat  necessitating  deep  excavation 
and  steeper  grades  for  the  larger  combined  sewers. 

c.  The  sanitary  sewers  must  be  placed  materially  deeper 
than  the  necessary  depth  for  the  storm  water  drains. 

d.  The  sewers  are  to  be  laid  in  rock,  necessitating  more 
difficult  excavation  for  the  larger  combined  sewers. 

e.  An  existing  sewerage  system  can  be  used  to  convey 
the  dry  weather  flow,  but  is  not  large  enough  for  the  storm 
sewage. 

/.  The  city  finances  are  such  that  the  greater  cost  of  the 
combined  system  cannot  be  met  and  sanitary  drainage  is 
imperative. 

g.  The  district  to  be  sewered  is  an  old  residential  section 
where  property  values  are  not  increasing  and  the  assess- 
ment must  be  kept  down. 

78 


PRELIMINARY  MAP  79 

Some  additional  points  given  in  a  report  by  Alvord  and  Burdick 
to  the  city  of  Billings,  Montana,  are: 

The  separate  system  of  sewerage  should  be  used,  where: 

1st.  Storm  water  does  not  require  extensive  under- 
ground removal,  or  where  it  can  be  concentrated  in  a  few 
shallow  underground  channels. 

2nd.  Drainage  areas  are  short  and  steep  facilitating 
rapid  flow  of  water  over  street  surfaces  to  the  natural  water 
courses. 

3rd.  The  sanitary  sewage  must  be  pumped. 

4th.  Sewers  are  being  built  in  advance  of  the  city's 
development  to  encourage  its  growth. 

5th.  The  existing  sewer  is  laid  at  grades  unsuitable  for 
sanitary  sewage,  it  can  be  used  as  a  storm  sewer. 

A  combined  system  must  be  relatively  larger  than  a 
separate  storm  sewer  as  the  latter  may  overflow  on  ex- 
ceptional occasions,  but  the  former  never. 

A  combined  system  of  sewerage  should  be  used  where : 
1st.  It  is  evident  that  storm  and  sanitary  sewerage 
must  be  provided  soon. 

2nd.  Both  sanitary  and  storm  sewage  must  be  pumped. 
3rd.  The  district  is  densely  built  up. 

42.  Preliminary  Map. — The  first  step  in  the  design  of  a  sewer- 
age system  is  the  preparation  of  a  map  of  the  district  to  be  served 
within  the  limits  of  its  probable  growth.  The  map  should  be  on 
a  scale  of  at  least  200  feet  to  the  inch  in  the  built  up  sections  or 
other  areas  where  it  is  anticipated  that  sewers  may  be  built,  and 
where  much  detail  is  to  be  shown  a  scale  as  large  as  40  feet  to  the 
inch  may  have  to  be  used.  The  adoption  of  so  large  a  scale  will 
usually  necessitate  the  division  of  the  city  or  sewer  district  into 
sections.  A  key  map  should  be  drawn  to  such  a  scale  that  the 
various  sections  represented  by  separate  drawings  can  all  be 
shown  upon  it.  In  preparing  the  enlarged  portions  of  the  map 
it  is  not  necessary  to  include  these  portions  of  the  city  in  which 
it  is  improbable  that  sewers  will  be  constructed,  such  as  parks  and 
cemeteries. 

The  contour  interval  should  depend  on  the  character  of  the 
district  and  the  slope  of  the  land.  In  those  sections  drawn  to  a 
scale  of  200  feet  to  the  inch  for  slopes  over  5  per  cent,  the  contour 
interval  need  not  be  closer  than  10  feet.  For  slopes  between  1  and 
5  per  cent  the  contour  interval  should  be  5  feet.  For  flatter 


80  DESIGN  OF  SEWERAGE  SYSTEMS 

slopes  the  interval  should  not  exceed  2  feet,  and  a  one  foot  interval 
is  sometimes  desirable.  In  general  the  horizontal  distances 
between  contours  should  not  exceed  400  feet  and  they  should  be 
close  enough  to  show  important  features  of  the  natural  drainage. 
Elevations  should  also  be  given  at  street  intersections,  and  at 
abrupt  changes  in  grade.  For  portions  of  the  map  on  a  smaller 
scale  the  contours  need  be  sufficiently  close  to  show  only  the 
drainage  lines  and  the  general  slope  of  the  land. 

The  following  may  be  shown  on  the  preliminary  map:  the 
elevation  of  lots  and  cellars;  the  character  of  the  built  up  districts, 
whether  cheap  frame  residences,  flat-roof  buildings,  manufacturing 
plants,  etc.;  property  lines;  width  of  streets  between  property 
lines  and  between  curb  lines;  the  width  and  character  of  the 
sidewalks  and  pavements;  street  car  and  railroad  tracks;  exist- 
ing underground  structures  such  as  sewers,  water  pipes,  telephone 
conduits,  etc.;  the  location  of  important  structures  which  may 
have  a  bearing  on  the  design  of  the  sewers  such  as  bridges,  rail- 
road tunnels,  deep  cuts,  culverts,  etc. ;  and  the  location  of  possible 
sewer  outlets  and  the  sites  for  sewage  disposal  plants. 

Fig.  24  shows  a  preliminary  map  for  a  section  of  a  city,  on 
which  the  necessary  information  has  been  entered.  The  map  is 
made  from  survey  notes.  All  streets  are  paved  with  brick.  The 
alleys  are  unpaved.  The  entire  section  is  built  up  with  high-class 
detached  residences  averaging  one  to  each  lot.  The  lots  vary  from 
1  to  3  feet  above  the  elevation  of  the  street. 

43.  Layout  of  the  Separate  System. — Upon  completion  of  the 
preliminary  map  a  tentative  plan  of  the  system  is  laid  out.  The 
lines  of  the  sewer  pipe  are  drawn  in  pencil,  usually  along  the  center 
line  of  the  street  or  alley  in  such  a  manner  that  a  sewer  will  be 
provided  within  50  feet  or  less  of  every  lot.  The  location  of  the 
sewers  should  be  such  as  to  give  the  most  desirable  combination 
of  low  cost,  short  house  connections,  proper  depth  for  cellar  drain- 
age, and  avoidance  of  paved  streets.  Some  dispute  arises  among 
engineers  as  to  the  advisability  of  placing  pipes  in  alleys,  although 
there  is  less  opposition  to  so  placing  sewers  than  any  other  utility 
conduit.  The  principal  advantage  in  placing  sewers  in  alleys  is 
to  avoid  disturbing  the  pavement  of  the  street,  but  if  both  street 
and  alley  are  paved  it  is  usually  more  economical  to  place  the 
sewer  in  the  street  as  the  house  connections  will  be  shorter.  On 
boulevards  and  other  wide  streets  such  as  Meridian  Avenue  in 


LOCATION  AND    NUMBERING  OF  MANHOLES  81 

Fig.  24,  the  sewers  are  placed  in  the  parking  on  each  side  of  the 
street,  rather  than  to  disturb  the  pavement  and  lay  long  house 
connections  to  the  center  of  the  street. 

All  pipes  should  be  made  to  slope,  where  possible,  in  the  direc- 
tion of  the  natural  slope  of  the  ground.  The  preliminary  layout 
of  the  system  is  shown  in  Fig.  24.  The  lowest  point  in  the  portion 
of  the  system  shown  is  in  the  alley  between  Alabama  and  Tennessee 
Streets.  The  flow  in  all  pipes  is  towards  this  point,  and  only 
one  pipe  drains  away  from  any  junction,  except  that  more  than 
one  pipe  may  drain  from  a  terminal  manhole  on  a  summit. 

44.  Location  and  Numbering  of  Manholes. — Manholes   are 
next  located  on  the  pipes  of  this  tentative  layout.     Good  practice 
calls  for  the  location  of  a  manhole  at  every  change  in  direction, 
grade,  elevation,  or  size  of  pipe,  except  in  sewers  60  inches  in  diam- 
eter or  larger.     The  manholes  should  not  be  more  than  300  to  500 
feet  apart,  and  preferably  as  close  as  200  to  300  feet.     In  sewers 
too  small  for  a  man  to  enter  the  distance  is  fixed  by  the  length  of 
sewer  rods  which   can  be  worked   successfully.     In  the  larger 
sewers  the  distances  are  sometimes  made  greater  but  inadvisedly 
so,  since  quick  means  of  escape  should  be  provided  for  workmen 
from  a  sudden  rise  of  water  in  the  sewer,  or  the  effect  of  an  asphyx- 
iating gas.     In  the  preliminary  layout  the  manholes  are  located 
at  pipe  intersections,  changes  in  direction,  and  not  over  300  to 
500  feet  apart  on  long  straight  runs  at  convenient  points  such  as 
opposite  street  intersections  where  other  sewers  may  enter. 

No  standard  system  of  manhole  numbering  has  been  adopted. 
A  system  which  avoids  confusion  and  is  subject  to  unlimited 
extension  is  to  number  the  manholes  consecutively  upwards  from 
the  outlet,  beginning  a  new  series  of  numbers  prefixed  by  some 
index  number  or  letter  for  each  branch  or  lateral.  This  system 
has  been  followed  with  the  manholes  on  Fig.  24. 

45.  Drainage  Areas. — The  quantity  of  dry  weather  sewage  is 
determined  by  the  population  rather  than  the  topography.     Lot 
lines  and  street  intersections  or  other  artificial  lines  marking  the 
boundaries  between  districts  are  therefore  taken  as  watershed 
lines  for  sanitary  sewers.     The  quantity  of  sewage  to  be  carried 
and  the  available  slope  are  the  determining  factors  in  fixing  the 
diameter  of  the  sewer.     Since  there  may  be  no  change  in  diameter 
or  slope  between  manholes  the  quantity  of  sewage  delivered  by  a 
sewer  into  any  manhole  will  determine  the  diameter  of  the  sewer 


82 


DESIGN  OF  SEWERAGE  SYSTEMS 


LAYOUT  OF  THE  SEPARATE  SYSTEM 


83 


QQ 


.3 

i 
i 


84  DESIGN  OF  SEWERAGE  SYSTEMS 

between  it  and  the  next  manhole  above.  In  order  to  determine 
the  additional  amount  contributed  between  manholes  a  line  is 
drawn  around  the  drainage  area  tributary  to  each  manhole. 
This  line  generally  follows  property  lines  and  the  center  lines  of 
streets  or  alleys,  its  position  being  such  that  it  includes  all  the 
area  draining  into  one  manhole,  and  excludes  all  areas  draining 
elsewhere.  An  entire  lot  is  usually  assumed  to  lie  within  the 
drainage  area  into  which  the  building  on  the  lot  drains.  In 
laying  out  these  areas  it  is  best  to  commence  at  the  upper  end  of  a 
lateral  and  work  down  to  a  junction.  Then  start  again  at  the 
upper  end  of  another  lateral  entering  this  junction,  and  continue 
thus  until  the  map  has  been  covered. 

The  areas  are  given  the  same  numbers  as  the  manholes  into 
which  they  drain.  The  dividing  lines  for  the  drainage  areas  on 
Fig.  24  are  shown  as  dot  and  dash  lines,  and  the  areas  enclosed  are 
appropriately  numbered.  If  more  than  one  sewer  drains  into 
the  same  manhole  the  area  should  be  subdivided  so  that  each 
subdivision  encloses  only  the  area  contributing  through  one 
sewer.  Such  a  condition  is  shown  at  manhole  C2.  The  areas 
are  designated  by  subletters  or  symbols  corresponding  to  the 
symbol  used  for  the  sewer  into  which  they  drain.  For  example, 
the  two  areas  contributing  to  manhole  C2  are  lettered  C2K  and 
C2D.  The  sewer  from  manhole  C3  to  C2  receives  no  addition,  it 
being  assumed  that  all  the  lots  adjacent  to  it  drain  into  the  sewer 
on  the  alley.  There  is  therefore  no  area  C2.  Likewise  there  is 
no  area  Ale- 

46.  Quantity  of  Sewage. — The  remaining  work  in  the  compu- 
tation of  the  quantity  of  sewage  is  best  kept  in  order  by  a  tabula- 
tion. Table  19  shows  the  computations  for  the  sewers  discharging 
from  the  east  into  manhole  No.  142.  The  computation  should 
begin  at  the  upper  end  of  a  lateral,  continue  to  a  junction,  and  then 
start  again  at  the  upper  end  of  another  lateral  entering  this  junc- 
tion. Each  line  in  the  table  should  be  filled  in  completely  from 
left  to  right  before  proceeding  with  the  computations  on  the  next 
line.  In  the  illustrative  solution  in  Table  19,  computations  for 
quantity  have  not  been  made  between  manholes  where  it  was 
apparent  that  there  would  be  an  insufficient  additional  quantity 
to  necessitate  a  change  in  the  size  of  the  pipe. 

In  making  these  computations  the  assumptions  of  quantity 
and  other  factors  given  below  indicate  the  sort  of  assumptions 


QUANTITY  OF  SEWAGE.  85 

which  must  be  made,  based  on  such  studies  as  are  given  in  Chapter 
III.  The  density  of  population  was  taken  as  20  persons  per  acre, 
the  assumption  being  based  on  the  census  and  the  character  of 
the  district.  The  average  sanitary  sewage  flow  was  taken  as 
100  gallons  per  capita  per  day.  The  per  cent  which  the  maximum 

500 
dry  weather  flow  is  of  the  average  was  taken  as  M  =  7^-,  in  which 

P  is  the  population  in  thousands.  The  per  cent  is  not  to  exceed 
500  nor  to  be  less  than  150.  The  rate  of  infiltration  of  ground 
water  was  assumed  as  50,000  gallons  per  mile  of  pipe  per  day. 

In  the  first  line  of  Table  19,  the  entries  in  columns  (1)  to  (6) 
are  self-explanatory.  There  are  no  entries  in  columns  (7)  to  (10), 
as  no  additional  sewage  is  contributed  between  manholes  3.5  and 
3.4.  In  column  (11),  2250  persons  are  recorded  as  the  number 
tributary  to  manhole  No.  3.5  in  the  district  to  the  north  and  west. 
These  people  contribute  an  average  of  100  gallons  per  person  per 
day,  or  a  total  of  0.346  second  foot.  This  quantity  is  entered  in 
column  (13).  The  figure  in  column  (14)  is  obtained  from  the 

500 
expression  M  =  p^.     Column  (15)  is  .01  of  the  product  of  columns 

(13)  and  (14).  Column  (16)  is  the  product  of  the  length  of  pipe 
between  manholes  3.5  and  3.4,  and  the  ground  water  unit  reduced 
to  cubic  feet  per  second.  Column  (17)  is  the  sum  of  column  (16), 
and  all  of  the  ground  water  tributary  to  manhole  3.5,  which  is 
not  recorded  in  the  table.  Column  (18)  is  the  sum  of  columns 
(15)  and  (17). 

No  new  principle  is  represented  in  the  second  and  third  lines. 

In  the  fourth  line  the  first  10  columns  need  no  further  explana- 
tion. The  (llth)  column  is  the  sum  of  the  (10th)  column,  and  the 
(llth)  column  in  the  third  line.  It  represents  the  total  number 
of  persons  tributary  to  manhole  3.4  on  lateral  No.  8.  Column 
(13)  in  the  fourth  line  is  the  sum  of  column  (13)  in  the  third  line 
and  the  (12th)  column  in  the  fourth  line,  and  the  (15th)  column 
in  the  fourth  line  is  the  product  of  the  2  preceding  columns  in  the 
fourth  line.  Note  that  in  no  case  is  the  figure  in  column  (15) 
the  sum  of  any  previous  figures  in  column  (15).  With  this  intro- 
duction the  student  should  be  able  to  check  the  remaining  figures 
in  the  table,  and  should  compute  the  quantity  of  sewage  entering 
manhole  No.  142  from  the  west,  making  reasonable  assumptions 
for  the  tributary  quantities  from  beyond  the  limits  of  the  map. 


86 


DESIGN  OF  SEWERAGE  SYSTEMS 


TABLE 

COMPUTATIONS  FOR  QUANTITY  OF  SEWAGE 


On  Street 

From  Street 

To  Street 

From 
Man- 
hole 

To 
Man- 
hole 

Length 
Feet 

Mark 
of 
Added 
Areas 

Nebraska  St  
Alley  S  of  Grant  St 

Map  margin  
Railroad 

AlleyS.  Grant  St.  .  . 
E   of  Missouri  St 

3.5 
8  3 

3.4 
8  2 

338 
328 

82 

Alley  S.of  Grant  St. 
Alley  S.  of  Grant  St. 
Nebraska  St  

Alley  S.  of  Meridian. 
Nebraska  St 

E.  of  Missouri  St  .  .  . 
E.  of  Kansas  St.  . 
AlleyS.  of  Grant  St. 

Railroad  
AlleyS   Meridian 

E.  of  Kansas  St  .  .  .  . 
Nebraska  St  
Alley  S.  of  Meridian. 

Nebraska  St  
Alley  S  of  Smith  Av 

8.2 
8.1 
3.4 

7.2 
3  3 

8.1 
3.4 
3.3 

3.3 
3  2 

355 
340 
380 

800 
304 

8.1 
3.48 

'7!!' 
3.37 

AlleyS  of  Smith  Ave 

Railroad 

Nebraska  St 

6  2 

3  2 

609 

6.1 
3  26 

Nebraska  St  
S  of  Cordovez  St 

Alley  S.of  Smith  Ave. 
Railroad 

S.  of  Cordovez  St.  .  . 
Nebraska  St 

3.2 
4   1 

3.1 
3   1 

300 
410 

3     14 

S.  of  CordovezSt..  . 
Nebraska  St 

Map  margin  
S  of  Cordovez  St 

Nebraska  St  
Long  St 

5.1 

3   1 

3.1 
148 

380 
172 

3.15 

Long  St  

Map  margin  

Nebraska  St  

149 

148 

380 

148 

Long  St 

Nebraska  St 

N   Carolina  St 

148 

147 

492 

Long  St  

N.  Carolina  St  

Georgia  St  

147 

146 

430 

Long  St          .... 

Georgia  St.  .  .        .    . 

Harris  St. 

146 

145 

419 

146 

Long  St 

Harris  St      

Tennessee  St  

145 

143 

725 

2.1 
143-145 

Column  No.       (1) 

(2) 

(3) 

(4) 

(5) 

(6) 

(7) 

*  Industrial  waste. 

TABLE 
COMPUTATIONS  FOR  SLOPE  AND  DIAMETER  OF 


On  Street 

From  Street 

To  Street 

From 
Man- 
hole 

To 
Man- 
hole 

Length, 
Feet 

Nebraska  St  
Alley  S  of  Grant  St 

Map  margin  

AlleyS.  of  Grant  St.. 
East  of  Missouri  St 

3.5 
8  3 

3.4 

8  2 

338 
328 

Alley  S.  of  Grant  St  . 
Alley  S.  of  Grant  St  . 
Nebraska  St  
Alley  S  of  Meridian 

East  of  Missouri  St  . 
East  of  Kansas  St.  .  . 
Alley  S.  of  Grant  St. 

East  of  Kansas  St..  .  . 
Nebraska  St  
Alley  S.  of  Meridian.  . 
Kansas  St 

8.2 
8.1 
3.4 
7  2 

8.1 
3.4 
3.3 

7  1 

355 
340 
380 
400 

Alley  S.  of  Meridian. 
Nebraska  St  
Alley  S.of  Smith  Ave. 
Alley  S.of  Smith  Ave. 
Nebraska  St  
Alley  S.  of  Cordovez. 

Kansas  St  
Alley  S.  of  Meridian. 
Railroad  
East  of  Kansas  St  .  . 
Alley  S.  of  Smith  Ave. 
Map  margin  

Nebraska  St  
Alley  S.  of  Smith  Ave. 
East  of  Kansas  St.  .  . 
Nebraska  St  
Alley  S.  of  Cordovez.  . 
Nebraska  St  
Nebraska  St 

7.1 
3.3 
6.2 
6.1 
3.2 
5.1 
4   1 

3.3 
3.2 
6.1 
3.2 
3.1 
3.1 
3   1 

400 
304 
305 
304 
300 
380 
410 

Nebraska  St  
Long  St 

Alley  S.  of  Cordovez. 

Long  St  
Nebraska  St 

3.1 
149 

148 
148 

172 
380 

Long  St  

Nebraska  St  

North  Carolina  St  .  .  . 

148 

147 

492 

Long  St  

North  Carolina  St  .  . 

Georgia  St  

147 

146 

430 

Long  St 

Georgia  St 

Harris  St  

146 

145 

419 

Alley  S.  of  JanisSt.  . 
Harris  St. 

End  of  Janis  St  
Alley  N  of  Janis  St 

Harris  St  
Long  St  

2.2 
2.1 

2.1 
145 

350 
135 

Long  St  
Long  St 

Harris  St  

Kentucky  St  

145 
144 

144 
143 

258 
282 

Tarbell  Ave 

Harris  St 

Long  St  

1.1 

143 

417 

Long  St 

Tennessee  St     . 

Alley  W.  of  Tenn.  St.. 

143 

142 

185 

Column  No.  (1) 

(2) 

(3) 

(4) 

(5) 

(6) 

QUANTITY  OF  SEWAGE 


87 


19 

FOB  A  SEPARATE  SEWERAGE  SYSTEM 


Cumu- 

Per 

Area, 
Acres 

Popu- 
lation 
per 
Acre 

Num- 
ber 
of 
Per- 
sons 

Total 
Per- 
sons 
Tribu- 
tary 

Avg. 
Sani- 
tary 
Flow, 
C.F.S. 

lative 
Avg. 
Sani- 
tary 
Flow, 

cent 
Max. 

Sani- 
tary 
is  of 

Total 
Max. 
Sani- 
tary, 
C.F.S. 

Incre- 
ment 
of 
Ground 
Water, 
CTT  cj 

Cumu- 
lative 
Ground 
Water, 
C.F.S. 

Total 
Flow, 
C.F.S. 

Line, 
Num- 
ber 

C.F.S. 

Average 

.JC  .  O. 

2250 

0.0000 

0.346 

425 

1.47 

0.005 

0.0187 

1.66 

1 

2.7 

'26 

'    54 

54 

.0084 

.0084 

500 

0.041 

.0048 

.0048 

0.046 

2 

3.41 

20 

68 

122 

.0106 

.0190 

500 

0.095 

.0052 

.010 

0.105 

3 

2.68 

20 

54 

176 

.0084 

.0274 

500 

0.137 

.0050 

.015 

0.152 

4 

2426 

.0000 

.373 

423 

1.58 

.0056 

.208 

1.79 

5 

7.14 

20 

142 

142 

.0221 

.0221 

500 

0.111 

.0117 

.0117 

0.123 

6 

2568 

.0000 

.395 

414 

1.63 

.0045 

.224 

1.85 

7 

3.82 

20 

76 

76 

.0119 

.0119 

500 

0.060 

.0089 

.0089 

0.069 

8 

2644 

.0000 

.407 

414 

1.68 

.0044 

.237 

1.92 

9 

3.io 

'26 

"62 

62 

.0096 

.0096 

500 

0.048 

.006 

.006 

0.054 

10 

2.69 

20 

54 

54 

.0084 

.0084 

500 

0.042 

.0056 

.0056 

0.048 

11 

2760 

.0000 

.425 

409 

1.74 

.0025 

.251 

1.99 

12 

1.53 

20 

31 

31 

.0048 

.0048 

500 

0.024 

.0056 

.0056 

0.030 

13 

2791 

.0000 

.430 

409 

1.76 

.0072 

.264 

2.02 

14 

2791 

1  .  000* 

.430 

409 

1.76 

.0064 

1.27 

3.03 

15 

0.81 

'26 

"ie 

2807 

.0025 

.433 

407 

1.76 

.0061 

1.28 

3.04 

16 

6.6 

20 

132 

2936 

.0205 

.454 

403 

1.83 

.024 

1.30 

3.13 

17 

(8) 

(9) 

(10) 

(11) 

(12) 

(13) 

(14) 

(15) 

(16) 

(17) 

(18) 

Treated  as  ground  water. 


20 

PIPES  FOR  A  SEPARATE  SEWERAGE  SYSTEM 


El.  of  Surface 

rp     4.      | 

Dia. 

Velocity 

Capacity 

El.  of  Invert 

Upper 
Man- 
hole 

Lower 
Man- 
hole 

l  otal 
Flow, 
C.F.S. 

Slope 

of 
Pipe, 
Inches 

wncn 
Full, 
Ft.  per 
Second 

when 
Full, 
Second 
Feet 

Manhole 

Lower 
Manhole 

Line 
Number 

105.8 

102.4 

1.66 

0.0108 

10 

3.25 

1.78 

97.80 

94.40 

1 

113.5 

112.0 

0.046 

.00575 

8 

2.00 

0.71 

105.50 

103  .  62 

2 

112.0 

107.7 

0.105 

.0110 

8 

2.78 

0.98 

103.61 

99.70 

3 

107.7 

102.4 

0.152 

.0156 

8 

3.27 

1.18 

99.69 

94.40 

4 

102.4 

100.7 

1.79 

.00385 

12 

2.28 

.79 

94.07 

92.61 

5 

111.8 

107.0 

.0120 

8 

2.90 

.03 

103.80 

99.00 

6 

107.0 

100.7 

'6:i23' 

.0157 

8 

3.28 

.18 

98.99 

92.70 

7 

100.7 

99.3 

1.85 

.0042 

12 

2.36 

.85 

92.37 

91.09 

8 

109.3 

105.3 

.0131 

8 

3.00 

.08 

101  .  30 

97.30 

9 

105.3 

99.3 

'6.'  069 

.0197 

8 

3.70 

.32 

97.29 

91.30 

10 

99.3 

101.1 

1.92 

.00213 

15 

2.00 

2.45 

90.84 

90.20 

11 

100.8 

101.1 

.00574 

8 

2.00 

0.71 

92.80 

90.62 

12 

104.6 

101.1 

'oiosi' 

.00854 

8 

2.46 

0.87 

96.60 

93.10 

13 

101   1 

98.7 

1.99 

.00213 

15 

2.00 

2.45 

90.04 

89.87 

14 

103.8 

98.7 

0.030 

.0134 

8 

3.04 

1.08 

95.80 

90.70 

15 

98.7 

103.8 

2.02 

.00213 

15 

2.00 

2.45    . 

89.86 

88.94 

16 

103.8 

99.1 

3.03 

.0016 

18 

2.00 

3.50 

88.69 

88.00 

17 

99.1 

96.9 

3.04 

.0016 

18 

2.00     i     3.50 

87.99 

87.32 

18 

105.2 

98.1 

.0203 

8 

3  .  78           1  .  35 

97.20 

90.10 

19 

98.1 

96.9 

.0088 

8 

2.53 

0.89 

90.09 

88.90 

20 

96.9 

94.4 

.00353 

18 

2.98 

5.20 

87.31 

86.40 

21 

94  4 

92  6 

00635 

18 

4  00 

7.00 

86.39 

84  '.  60 

22 

98.7 

92.6 

.0146 

8 

3.18 

1.14 

90  70 

84  60 

23 

92.6 

92.3 

3.13 

.0016 

18 

2.00 

3.50 

83.77 

83^47 

24 

(7) 

(8) 

(9) 

(10) 

(11) 

(12) 

(13) 

(14) 

(15) 

88  DESIGN  OF  SEWERAGE  SYSTEMS 

47.  Surface  Profile.— A  profile  of  the  surface  of  the  ground 
along  the  proposed  lines-  of  the  sewers  should  be  drawn  after  the 
completion  of  the  computations  for  quantity.     An  example  of  a 
profile  is  shown  in  Fig.  26  for  the  line  between  manholes  No.  3.5 
and  No.  147.     The  vertical  scale  should  be  at  least  10  times  the 
horizontal.     A  horizontal  scale  of  1  inch  to  200  feet  can  be  used 
where  not  much  detail  is  to  be  shown,  but  a  scale  of  one  1  to 
100  feet  is  more  common  and  more  satisfactory  and  even  one  inch 
to  10  feet  has  been  used.     The  information  to  be  given  and  the 
method  of  showing  it  are  illustrated  on  Fig.   26.     The  profile 
should  show  the  character  of  the  material  to  be  passed  through 
and  the  location  of  underground  obstacles  which  may  be  encoun- 
tered.    The  method  of  obtaining  this  information  is  taken  up  in 
Chapter  II.     The  collection  of  the  information  should  be  com- 
pleted as  far  as  possible  previous  to  design,  and  borings  and  other 
investigations  made  as  soon  as  the  tentative  routes  for  the  sewers 
have  been  selected. 

48.  Slope  and  Diameter  of  Sewers. — After  the  quantity  of 
sewage  to  be  carried  has  been  determined,  and  the  profile  of  the 
ground  surface  has  been  drawn,  it  is  possible  to  determine  the 
slope  and  diameter  of  the  sewer.     A  table  such  as  No.  20  is  made 
up  somewhat  similar  to  No.  19,  or  which  may  be  an  extension  of 
Table  19  since  the  first  6  columns  in  both  tables  are  the  same. 
The  elevation  of  the  surface  at  the  upper  and  lower  manholes  is 
read  from  the  profile. 

The  depth  of  the  sewer  below  the  ground  surface  is  first 
determined.  Sewers  should  be  sufficiently  deep  to  drain  cellars 
of  ordinary  depth.  In  residential  districts  cellars  are  seldom  more 
than  5  feet  below  the  ground  surface.  To  this  depth  must  be 
added  the  drop  necessary  for  the  grade  of  the  house  sewer.  Six- 
inch  pipe  laid  on  a  minimum  grade  of  1.67  per  cent  is  a  common 
size  and  slope  restriction  for  house  drains  or  sewers.  An  addi- 
tional 12  inches  should  be  allowed  for  the  bends  in  the  pipe  and 
the  depth  of  the  pipe  under  the  cellar  floor.  Where  the  eleva- 
tion of  the  street  and  lots  is  about  the  same,  and  the  street  is  not 
over  80  feet  in  width  between  property  lines,  a  minimum  depth 
of  8  feet  to  the  invert  of  sewers,  24  inches  or  less  in  diameter  is 
satisfactory.  This  is  on  the  assumption  that  the  axes  'of  the 
house  drain  and  the  sewer  intersect.  For  larger  pipes  the  depth 
should  be  increased  so  that  when  the  street  sewer  is  flowing  full, 


SURFACE   PROFILE 


89 


90  DESIGN  OF  SEWERAGE  SYSTEMS 

sewage  will  not  back  up  into  the  cellars  or  for  any  great  distance 
into  the  tributary  pipes. 

The  grade  or  slope  at  which  a  sewer  shall  lie  may  be  fixed  by: 
the  slope  of  the  ground  surface;  the  minimum  permissible  self- 
cleansing  velocity;  a  combination  of  diameter,  velocity,  and 
quantity;  or  the  maximum  permissible  velocity  of  flow.  Sewers 
are  laid  either  parallel  to  the  ground  surface  where  the  slope  is 
sufficient  or  where  possible  without  coming  too  near  the  surface 
they  are  laid  on  a  flatter  grade  to  avoid  unnecessary  excavation. 
The  minimum  permissible  slope  is  fixed  by  the  minimum  permis- 
sible velocity. 

The  velocity  of  flow  in  a  sewer  should  be  sufficient  to  prevent 
the  sedimentation  of  sludge  and  light  mineral  matter.  Such  a 
velocity  is  in  the  neighborhood  of  1  foot  per  second.  Since 
sewers  seldom  flow  full  this  velocity  should  be  available  under 
ordinary  conditions  of  dry  weather  flow.  The  minimum  velocity 
when  full  should  therefore  be  about  2  feet  per  second.  Under 
this  condition,  the  velocity  of  1  foot  per  second  is  not  reached 
until  the  sewer  is  less  than  18  per  cent  full.  The  velocity  in  small 
sewers  should  be  made  somewhat  faster  than  in  large  sewers 
since  the  velocity  of  flow  for  small  depths  in  small  pipes  is  less 
than  for  the  same  proportionate  depth  in  large  pipes.  The 
maximum  permissible  velocity  of  flow  is  fixed  at  about  10  feet 
per  second  in  order  to  avoid  excessive  erosion  of  the  invert.  If  the 
sewer  is  carefully  laid  this  limit  may  be  exceeded  in  sanitary  sewers. 

The  method  for  determining  the  grade  and  diameter  of  sewers 
is  best  explained  through  an  illustrative  problem  which  is  worked 
out  in  Table  20  for  the  profile  shown  on  Fig.  26.  The  figures 
are  inserted  in  the  table  from  left  to  right  in  each  line,  one  line 
being  completed  before  the  next  one  is  commenced.  The  head- 
ings in  the  first  6  columns  are  self-explanatory.  The  elevations 
of  the  surface  at  the  upper  and  lower  manholes  are  read  from  the 
profile.  The  total  flow  is  read  from  column  (18)  in  Table  19. 
The  slope  of  the  ground  surface  is  then  computed,  and  with  the 
quantity,  slope,  and  coefficient  of  roughness,  the  diameter  of  the 
pipe  and  the  velocity  of  flow  are  read  from  Fig.  15. 

The  following  conditions  may  arise: 

(1)  The  diameter  required  is  less  than  8  inches.  Use  a 
diameter  of  8  inches  as  experience  has  shown  that  the  use  of 
smaller  diameters  is  unsatisfactory. 


SLOPE   AND  DIAMETER  OF  SEWERS  91 

(2)  The  velocity  of  flow  when  the  sewer  is  full  is  less  than 
2  feet  per  second.     Increase  the  slope  until  the  velocity 
when  full  is  2  feet  per  second. 

(3)  The  diameter  of  the  pipe  required  is  not  one  of  the 
commercial  sizes  shown  in  Fig.  15.     Use  the  next  largest 
commercial  size. 

(4)  The  slope  of  the  ground  surface  is  steeper  than 
necessary   to   maintain    the    required   minimum   velocity 
and  the  upper  end  of  the  sewer  is  deeper  than  the  required 
minimum  depth.     Place  the  sewer  on  the  minimum  per- 
missible grade,  or  upon  such  a  grade  that  its  lower  end 
will  be  at  the  minimum  permissible  depth. 

(5)  The  slope  of  the  ground  surface  is  so  steep  as  to 
make  the  velocity  of  flow  greater  than  the  maximum  rate 
permissible.     Reduce  the  grade  by  deepening  the  sewer  at 
the  upper  manhole  and  using  a  drop  manhole  at  this  point. 

It  is  not  permissible  to  use  a  pipe  larger  than  that  called  for 
by  the  above  conditions.  This  is  attempted  sometimes  in  order 
to  reduce  the  grade  and  thereby  save  excavation,  under  the  rule 
of  a  minimum  velocity  of  2  feet  per  second  when  full.  It  is 
better  to  use  the  smaller  pipe  on  the  flat  grade  as  the  quantity  of 
sewage  is  insufficient  to  fill  the  larger  sewer  and  the  minimum 
permissible  velocity  is  more  quickly  reached. 

Having  determined  the  slope,  the  diameter,  and  the  capacity 
of  the  pipe  to  be  used,  these  values  are  entered  in  the  table. 
The  elevations  of  the  invert  of  the  pipe  at  the  upper  and  lower 
manholes  are  next  computed  and  entered  in  the  table.  This 
method  is  followed  until  all  of  the  diameters,  slopes,  and  eleva- 
tions have  been  determined. 

The  slopes  are  computed  from  center  to  center  of  manholes, 
but  an  extra  allowance  of  0.01  of  a  foot  is  allowed  by  some  design- 
ers for  the  increased  loss  in  head  in  passing  through  the  manhole. 
When  it  becomes  necessary  to  increase  the  diameter  of  the  sewer 
the  top  of  the  outgoing  sewer  is  placed  at  the  same  elevation  or 
below  the  top  of  the  lowest  incoming  sewer.  No  extra  allow- 
ance is  made  to  compensate  for  loss  in  head  in  the  manhole  in  this 
case.  This  case  is  illustrated  in  columns  (14)  and  (15)  in  lines 
(16)  and  (17)  of  Table  20.  All  of  the  conditions  listed  above  are 
illustrated  in  Table  20,  except  the  condition  for  a  velocity  greater 
than  10  feet  per  second. 

The  first  condition  is  met  at  the  head  of  practically  every 
lateral,  and  is  illustrated  in  the  second  line. 


92  DESIGN  OF  SEWERAGE  SYSTEMS 

The  second  condition  is  also  illustrated  in  the  second  line. 
The  slope  of  the  ground  surface  is  0.0046,  which  gives  a  velocity 
of  only  1.8  feet  per  second  in  an  8-inch  pipe.  The  slope  is  there- 
fore increased  to  0.00575,  on  which  the  full  velocity  is  2  feet  per 
second. 

The  third  condition  is  met  in  the  first  line.  The  diameter 
called  for  to  carry  1.66  cubic  feet  per  second  on  a  slope  of  0.0108 
is  slightly  less  than  10  inches.  A  10-inch  pipe  is  therefore  used 
and  its  full  capacity  and  velocity  are  recorded. 

The  fourth  condition  is  illustrated  in  the  fourteenth  line. 
The  cut  at  manhole  No.  3.1  is  11.1  feet.  The  slope  of  the  ground 
is  0.014,  much  steeper  than  is  necessary  to  maintain  the  minimum 
velocity  in  a  15-inch  pipe.  The  pipe  is  therefore  placed  on  the 
minimum  permissible  slope,  and  excavation  is  saved.  The  student 
should  check  the  figures  in  Table  20  and  be  sure  that  they  are 
understood  before  an  attempt  is  made  to  make  a  design  inde- 
pendently. 

49.  The  Sewer  Profile. — The  profile  is  next  completed  as 
shown  in  Fig.  26,  the  pipe  line  being  drawn  in  as  the  computa- 
tions are  made.  The  cut  is  recorded  to  the  nearest  ^th  of  a  foot 
at  each  manhole,  or  change  in  grade.  It  should  not  be  given  else- 
where as  it  invites  controversy  with  the  contractor.  The  cut  is 
the  difference  of  the  elevation  of  the  invert  of  the  lowest  pipe  in 
the  trench  at  the  point  in  question,  and  the  surface  of  the  ground. 

The  stationing  should  be  shown  to  the  nearest  roth  of  a  foot. 
It  should  commence  at  0+00  at  the  outlet  and  increase  up  the 
sewer.  The  station  of  any  point  on  the  sewer  may  show  the  dis- 
tance from  it  to  the  outlet,  or  a  new  system  of  stationing  may  be 
commenced  at  important  junctions  or  at  each  junction. 

Elevations  of  the  surface  of  the  ground  should  be  shown  to 
the  nearest  iVth  of  a  foot,  and  the  invert  elevation  to  the  nearest 
i^-oth  of  a  foot. 

Only  the  main  line  sewer  is  shown  in  profile  in  Fig.  26.  The 
profiles  of  the  laterals  computed  in  Table  20,  have  not  been  shown. 
The  approximate  location  of  all  house  inlets  are  shown  on  the 
profile  and  located  exactly,  and  are  made  a  matter  of  record 
during  construction. 


PLANNING  THE  SYSTEM  93 

DESIGN  OF  A  STORM  WATER  SEWER  SYSTEM 

50.  Planning  the  System. — Storm  sewer  systems  are  seldom 
as  extensive  as  separate  or  combined  sewer  systems,  since  storm 
sewage  can  be  discharged  into  the  nearest  suitable  point  in  a 
flowing  stream  or  other  drainage  channel,  whereas  dry  weather 
or  combined  sewage  must  be  conducted  to  some  point  where  its 
discharge   will   be   inoffensive.     The   need   of   a    comprehensive 
general  plan  of  a  storm  sewer  system  is  quite  as  great,  however, 
as  for  a  separate  system.     The  haphazard  construction  of  sewers 
at  the  points  most  needed  for  the  moment  results  in  the  duplica- 
tion of  forgotten  drains,  expense  in,  increasing  the  capacity  of 
inadequate  sewers,  and  difficult  construction  due  to  underground 
structures  thoughtlessly  located.     A  comprehensive  plan  permits 
the  construction  of  sewers  where  they  are  needed  as  they  are 
required,  and  enables  all  probable  future  needs  to  be  cared  for  at  a 
minimum  of  expense. 

The  same  preliminary  survey,  map,  and  underground  informa- 
tion are  necessary  for  the  design  of  a  storm  sewer  system  as  for  a 
separate  sewer  system.  The  map  shown  on  Fig.  25  has  been  used 
for  the  design  of  a  storm-water  sewer  system. 

The  steps  in  the  design  of  a  storm- water  sewer  system  are: 
1st.  Note  the  most  advantageous  points  to  locate  the  inlets 
and  lay  out  the  system  to  drain  these  inlets.  2nd.  Determine 
the  required  capacity  of  the  sewers  by  a  study  of  the  run-off  from 
the  different  drainage  areas.  3rd.  Draw  the  profile  and  compute 
the  diameter  and  slope  of  the  pipes  required. 

51.  Location  of  Street  Inlets. — The  location  of  storm  sewers 
is  determined  mainly  by  the  desirable  location  of  the  street  inlets. 
The  inlets  must  therefore  be  located  before  the  system  can  be 
planned.     In  general  the  inlets  should  be  located  so  that  no  water 
will  flow  across  a  street  or  sidewalk,  in  order  to  reach  the  sewer. 
This  requires  that  inlets  be  placed  on  the  high  corners  at  street 
intersections,  in  depressions  between  street  intersections,  and  at 
sufficiently  frequent  intervals  that  the  gutters  may  not  be  over- 
loaded.    City  blocks  are  seldom  so  long  as  to  necessitate  the  loca- 
tion of  inlets  between  crossings  solely  on  account  of  inadequate 
gutter  capacity.     The  capacity  of  a  gutter  can  be   computed 
approximately  by  the  application   of   Kutter's  formula.     Inlet 
capacities  are  discussed  in  Chapter  VI.     When  the  area  drained 


94  DESIGN  OF  SEWERAGE  SYSTEMS 

is  sufficiently  large  to  tax  the  capacity  of  the  gutter  or  inlet,  an 
inlet  should  be  installed  regardless  of  the  location  of  the  street 
intersections. 

The  street  inlets  are  located  on  the  map  as  shown  in  Fig.  25. 
The  sewer  lines  are  then  located  so  as  to  make  the  length  of  pipe 
to  pass  near  to  all  inlets  a  minimum.  Storm  sewers  are  seldom 
placed  near  the  center  of  a  street  because  of  the  frequent  crowded 
condition  on  this  line. 

52.  Drainage  Areas. — The  outline  of  a  drainage  area  is  drawn 
so  that  all  water  falling  within  the  area  outlined  will  enter  the 
same  inlet,  and  water  falling  on  any  point  beyond  the  outline  will 
enter  .some  other  inlet.  This  requires  that  the  outline  follow 
true  drainage  lines  rather  than  the  artificial  land  divisions  used 
in  locating  the  drainage  lines  in  the  design  of  sanitary  sewers. 
The  drainage  lines  are  determined  by  pavement  slopes,  location 
of  downspouts,  paved  or  unpaved  yards,  grading  of  lawns  and 
the  many  other  features  of  the  natural  drainage  which  are  altered 
by  the  building  up  of  a  city.  The  location  of  the  drainage  lines 
is  fixed  as  the  result  of  a  study  of  local  conditions. 

The  watershed  or  drainage  lines  are  shown  on  Fig.  25  by  means 
of  dot  and  dash  lines.  A  drainage  line  passes  down  the  middle  of 
each  street  because  the  crown  of  the  street  throws  the  water  to  either 
side  and  directs  it  to  different  inlets.  A  watershed  line  is  drawn 
about  50  feet  west  of  such  streets  as  Kentucky  St.,  Florida  St.,  etc., 
because  the  downspouts  from  the  houses  on  those  streets  discharge 
or  will  discharge  into  the  street  on  which  they  face.  The  location 
of  any  watershed  line  within  20  feet  more  or  less  is,  in  most 
cases,  a  matter  of  judgment  rather  than  exactness.  Each  area 
is  given  an  identifying  number  or  mark  which  is  useful  only  in 
design.  It  usually  corresponds  to  the  inlet  number. 

63.  Computation  of  Flood  Flow  by  McMath  Formula. — 
McMath's  Formula  is  used  as  an  example  of  the  method  pursued 
when  an  empirical  formula  is  adopted  for  the  computation  of 
run-off,  and  because  of  its  frequent  use  in  practice.  Other  formu- 
las may  be  more  satisfactory  under  favorable  conditions. 

Computations  should  be  kept  in  order  by  a  tabulation  such  as 
is  shown  in  Table  21,  in  which  the  quantity  of  storm  flow  discharged 
from  the  sewer  at  the  foot  of  Tennessee  St.,  on  Fig.  25,  has  been 
computed  by  means  of  the  McMath  Formula,  using  the  constants 
suggested  for  St.  Louis  conditions,  z  =  2.75,  and  c=0.75.  The 


FLOOD   FLOW  BY  RATIONAL  METHOD  95 

solutions  of  the  formula  have  been  made  by  means  of  Fig.  11. 
The  column  headings  in  the  Table  are  explanatory  of  the  figures 
as  recorded.  The  computation  should  begin  at  the  upper  end 
of  a  lateral,  proceed  to  the  first  junction  and  then  return  to  the 
head  of  another  lateral  tributary  to  this  junction.  They  should 
be  continued  in  the  same  manner  until  all  tributary  areas  have 
been  covered.  Special  computations  will  be  necessary  for  the 
determination  of  the  maximum  quantity  of  storm  water  entering 
each  inlet  to  avoid  the  flooding  of  an  inlet  or  gutter.  These 
computations  have  not  been  shown  as  they  are  so  easily  made  by 
the  application  of  McMath's  Formula  to  each  area  concerned. 

The  determination  of  the  average  slope  ratio  is  a  matter  of 
judgment,  based  on  the  average  natural  slope  of  the  surface  of 
the  ground  and  an  estimate  of  the  probable  future  conditions. 

54.  Computation  of  Flood  Flow  by  Rational  Method.— The 
rational  method  for  the  computation  of  storm  water  run-off  is 
described  in  Chapter  III.  An  example  of  its  application  to  storm 
sewer  design  is  given  here  for  the  district  shown  in  Fig.  25.1 
The  computations  are  shown  in  Table  21.  As  in  the  preceding 
designs  the  table  has  been  filled  in  from  left  to  right  and  line  by 
line.  Computations  have  started  at  the  upper  end  of  laterals 
tributary  to  each  junction.  The  column  headed  /  represents  the 
imperviousness  factor  in  the  expression  Q=AIR.  It  is  based  on 
judgment  guided  by  the  constants  given  in  Chapter  III  concern- 
ing imperviousness.  The  column  headed  "  Equivalent  100  per 
cent  I  acres  "  is  the  product  of  the  two  preceding  columns.  It 
reduces  all  areas  to  the  same  terms  so  that  they  can  be  added  for 
entry  in  the  column  headed  "  Total  100  per  cent  /  acres."  It 
may  be  necessary  to  record  the  values  for  this  column  on  several 
lines  where  the  imperviousnesses  of  the  tributary  areas  are  differ- 
ent. This  condition  is  illustrated  in  the  last  line  of  the  table, 
for  the  length  of  sewer  nearest  the  outlet.  In  the  preceding  lines 
the  imperviousness  recorded  represents  an  average  for  all  the 
tributary  areas. 

The  time  of  concentration  in  minutes  is  assumed  by  judgment 
for  the  first  area.  For  all  subsequent  areas  it  is  the  sum  of  the 
time  of  concentration  for  the  area  or  areas  tributary  to  the  inlet 
next  above  and  the  time  of  flow  in  the  sewer  from  the  inlet  next 

1  For  diagrams  for  the  Solution  of  the  Rational  Method,  see  Eng.  News- 
Record,  Vol.  83,  1919,  p.  868  and  Vol.  85,  1920,  p.  151. 


96 


DESIGN  OF  SEWERAGE  SYSTEMS 


TABLE 

COMPUTATIONS  FOR  THE  QUANTITY  OF  STORM  SEWAGE 


By  Me  Math's  Formula 

Identifying 

s 

S 

GQ 

On  Street 

From  Street 

To  Street 

Number  of  Areas 

4< 

«2 
b 

6 

Drained 

"3"? 

h 

a 

|l 

<s 

1—  1  * 

o 

to 
0 

•Sg 

•^Q 

a 

d 

3 

£ 

1 

M 

State  

N.  Carolina  

S.  Carolina.  . 

91  and  92 

2.35 

2.35 

0.005 

5.5 

State  

S.  Carolina  

Georgia  

88,  89  and  90 

3.0 

5.35 

.005 

10.8 

State  

Georgia  

Florida  

85,  86  and  87 

3.0 

8.35 

.007 

16.5 

State  

Florida  

Kentucky  .  .  . 

81,  83  and  84 

3.0 

11.35 

.009 

22.0 

State  

Kentucky  

Tennessee.  .  . 

79,  80  and  82 

3.0 

14.35 

.010 

28.0 

State  

Texas     

Louisiana.  .  .  . 

76  and  others 

3.8 

3.8 

.005 

8.3 

State  

Louisiana  

Alabama  .... 

73,  74  and  75 

3.7 

7.5 

.007 

15.0 

State  

Alabama  

Tennessee.  .  . 

70,  71  and  72 

3.0 

10.5 

.006 

19.0 

Tennessee.  . 

State  

Talon  

68,  69,  77  and  78 

4.3 

29.15 

.15 

52 

Talon  

Albemarle  

Tennessee  .  .  . 

65,  66  and  67 

2.8 

2.8 

.018 

8.4 

Tennessee.  . 

Talon  

Burnside  .... 

64  and  64a 

0.7 

29.85 

.15 

55 

Burnside  .  .  . 

N.  Carolina  

S.  Carolina.  . 

57,  58  and  59 

2.84 

2.84 

.008 

7.2 

Burnside.  .  . 

S.  Carolina  

Georgia  

54,  55  and  56 

3.88 

6.72 

.010 

14.9 

Burnside.  .  . 

Georgia.  

Florida  

50,  52  and  53 

3.88 

10.60 

.012 

22 

Burnside  . 

Florida 

Kentucky  . 

47,  48  and  51 

3.88 

14.48 

.013 

30 

Burnside  . 

Kentucky 

Tennessee  . 

44,  45  and  46 

3.88 

18.36 

.013 

36 

Tennessee  .  . 

Burnside  

Elm  

42  and  43 

2.84 

51.05 

.015 

82 

Elm  

Above  Chetwood  . 

Chetwood.  .  . 

Included  in  next  line  below 

Elm  

Chetwood 

\lbemarle 

31,  32  and  33 

2.75 

2.75 

.007 

7.0 

Elm  

Albemarle 

Tennessee  .  .  . 

27,  28,  29  and  30 

5.75 

8.50 

.016 

20 

Tennessee  . 

Elm 

Varennes  .... 

25,  26  and  41 

2.62 

62.17 

.017 

100 

Varennes.  .  . 

S   Carolina  

Georgia  

17,  18  and  19 

3.17 

3.17 

.010 

8.3 

Varennes 

Georgia 

Florida 

14,  15  and  16 

3.17 

6.34 

.011 

14.5 

Varennes.  .  . 

Florida  

Kentucky.  .  . 

11,  12  and  13 

3.17 

9.51 

.013 

21 

Varennes.  .  . 

Kentucky  

Tennessee.  .  . 

8,  9  and  10 

3.17 

12.68 

.013 

26 

Tennessee.  . 

Varennes  

Boulevard  .  .  . 

6  and  7 

2.32 

77.17 

.017 

120 

Tennessee.  . 

Boulevard  

Outlet 

1,  2,  3,  4  and  5 

4.72 

81.89 

.017 

122 

above  to  the  inlet  in  question.  For  example,  in  line  2  the  time 
8.1  minutes  is  the  sum  of  7.0  minutes  time  of  concentration  to 
the  inlet  at  the  corner  of  State  and  North  Carolina  St.,  and  the 
time  of  flow  of  1.1  minute  in  the  sewer  on  State  St.  from  North 
Carolina  St.  to  South  Carolina  St.  Where  two  sewers  are  con- 
verging as  at  the  corner  of  Varennes  Road  and  Tennessee  St.  the 
longest  time  is  taken.  For  example,  the  time  of  concentration 


FLOOD  FLOW  BY  RATIONAL  METHOD 


97 


21 

AT  THE  FOOT  OF  TENNESSEE  STREET  ON  FlGURE  25 


By  Rational  Method 

Line  Number 

! 

o 

-< 

1 

I 

Equivalent 
100  Per  Cent 
I  Acres 

ll 

§H 

I-H  *J 

|l 

Time  of  Con- 
centration, 
Minutes 

R 

Q 

S 

V 

js 

M 
C 
O 

fcl 

Ufa 

eg 

1 

a; 
02 

C 

I 

£ 

2.35 

0.50 

1.17 

1.17 

7.0 

.8 

5.6 

0.011 

4.6 

300 

1.1 

1 

3.00 

.50 

1.50 

2.67 

8.1 

.6 

12.2 

.010 

5.5 

300 

0.9 

2 

3.00 

.50 

1.50 

4.17 

9.0 

.4 

18.3 

.012 

5.8 

300 

0.9 

3 

3.00 

.50 

1.50 

5.67 

9.9 

.2 

23.9 

.009 

6.0 

300 

0.8 

4 

3.00 

.50 

1.50 

7.17 

10.7 

.1 

29.3 

.009 

6.2 

300 

0.8 

5 

3.80 

.35 

1.33 

1.33 

10.0 

.2 

5.6 

.009 

3.2 

370 

1.9 

6 

3.70 

.40 

1.48 

2.81 

11.9 

3.9 

11.0 

.011 

5.2 

300 

1.0 

7 

3.00 

.45 

1.35 

4.16 

12.9 

3.8 

15.8 

.002 

3.2 

300 

1.6 

8 

4.30 

.50 

2.15 

13.48 

14.5 

3.6 

48.5 

.019 

9.8 

450 

0.8 

9 

2.80 

.40 

1.12 

1.12 

8.0 

4.6 

5.2 

.004 

3.0 

210 

1.2 

10 

0.70 

.20 

0.14 

14.74 

15.3 

3.5 

51.5 

.006 

5.0 

120 

0.4 

11 

2.84 

.55 

1.56 

1.56 

10.0 

4.2 

6.5 

.008 

4.5 

300 

1.1 

12 

3.88 

.55 

2.13 

3.69 

11.1 

4.0 

14.8 

.007 

4.7 

300 

1.1 

13 

3.88 

.55 

2.13 

5.82 

12.2 

3.9 

22.7 

.011 

5.8 

300 

0.9 

14 

3.88 

.55 

2.13 

7.95 

13.1 

3.7 

29.4 

.016 

7.5 

300 

0.7 

15 

3.88 

.55 

2.13 

10.08 

13.8 

3.7 

37.3 

.019 

9.2 

300 

0.5 

16 

2.84 

.45 

2.28 

26.10 

15.7 

3.4 

88.8 

.015 

10.2 

280 

0.5 

17 

18 

2.75 

.40 

1.10 

1.10 

8.0 

4.6 

5.1 

.020 

5.3 

480 

1.5 

19 

5.75 

.45 

2.59 

3.69 

9.5 

4.3 

15.8 

.012 

6.1 

410 

1.1 

20 

2.62 

.50 

1.31 

30.00 

16.2 

3.4 

102 

.012 

10.2 

180 

0.3 

21 

3.17 

.55 

1.74 

1.74 

9.0 

4.4 

7.7 

.012 

5.2 

270 

0.9 

22 

3.17 

.55 

1.74 

3.48 

9.9 

4.2 

14.6 

.010 

5.7 

300 

0.9 

23 

3.17 

.55 

1.74 

5.22 

10.8 

4.1 

21.4 

.017 

7.7 

300 

0.6 

24 

3.17 

.55 

1.74 

6.96 

11.4 

4.0 

27.8 

.015 

7.8 

300 

0.6 

25 

2.32 
0.18 
1.38 
2.80 
0.36 

.55 
.80 
.50 
.55 
.75 

1.28 
0.14 
0.69 
1.54 
0.27 

32.84 
Area 
Area 
Areas 
35.48 

16.5 
No.  1 
No.  2 
No.  3 
16.9 

3.3 

108 

.012 

10.2 

230 

0.4 

26 
27 
28 
29 
30 

and  4 
3.3 

117 

Areas  No.  1-5  inclus 

ive 

down  Varennes  Road  to  Tennessee  St.  is  shown  in  line  25  as 
11.4+0.6=  12.0  minutes.  The  time  to  the  same  point  down  Ten- 
nessee St.  is  shown  in  line  21  as  16.2+0.3  =  16.5  minutes.  This 
time  is  therefore  used  in  line  26. 

R,  the  rate  of  rainfall  in  inches  per  hour  is  determined  by 
Talbot's  formula. 

Q,  is  in  cubic  feet  per  second  and  is  the  product  of  the  8th 


98  DESIGN  OF  SEWERAGE  SYSTEMS 

and  10th  columns.  Since  the  8th  column  is  the  sum  of  the  prod- 
ucts of  the  5th  and  the  6th  columns  for  the  lines  representing 
tributary  areas,  then  the  llth  column  is  the  product  of  A,  I,  and  R. 

S,  is  the  slope  on  which  it  is  assumed  that  the  sewer  will  be 
laid.  It  is  usually  assumed  as  parallel  to  the  ground  surface 
unless  the  velocity  for  this  slope  becomes  less  than  2  feet  per  sec- 
ond. In  such  a  case  the  slope  is  taken  as  one  which  will  cause 
this  velocity. 

V,  the  velocity  in  feet  per  second,  is  computed  from  diagrams 
for  the  solution  of  Kutter's  formula.  The  length  in  feet  is  scaled 
from  the  map  as  the  distance  between  inlets  or  groups  of  inlets, 
and  the  time  is  the  length  in  feet  divided  by  the  velocity  in  feet 
per  minute. 

Having  computed  the  quantity  of  flow  to  be  carried  in  the 
sewer,  the  design  is  completed  by  drawing  the  profile  and  com- 
puting the  diameters  and  slopes  by  the  same  method  as  used  in 
the  design  of  separate  sewers. 


CHAPTER  VI 
APPURTENANCES 

55.  General. — The  appurtenances  to  a  sewerage  system  are 
those  devices  which,  in  addition  to  the  pipes  and  conduits,  are 
essential  to  or  are  of  assistance  in  the  operation  of  the  system. 
Under  this  heading  are    included    such  structures   and    devices 
as:  manholes,  lampholes,  flush-tanks,  catch-basins,  street  inlets, 
regulators,  siphons,  junctions,  outlets,  grease  traps,  foundations 
and  underdrains. 

56.  Manholes. — A  manhole  is  an  opening  constructed  in  a 
sewer,  of  sufficient  size  to  permit  a  man  to  gain  access  to  the  sewer. 
Manholes  are  the  most  common  appurtenances  to  sewerage  sys- 
tems and  are  used  to  permit  inspection  and  the  removal  of  obstruc- 
tions from  the  pipes.     The  details  of  the  Baltimore  standard 
manholes  are  shown  in  Fig.  27  and  a  manhole  on  a  large  sewer  in 
Omaha  is  shown  in  Fig.  28.     The  features  of  these  designs  which 
should  be  noted  are  the  size  of  the  opening  and  working  space, 
and  the  strength  of  the  structure.     Manhole  openings  are  seldom 
made  less  than  20  inches  in  diameter  and  openings  24  inches  in 
diameter  are  preferable.     A  man  can  pass  through  any  opening 
that  he  can  get  his  hips  through  provided  he  can  bend  his  knees 
and  twist  his  shoulders  immediately  on  passing  the  hole.     For  this 
reason  the  manhole  should  widen  out  rapidly  immediately  below 
the  opening,  as  shown  in  Fig.  27  and  38. 

The  walls  of  the  manhole  may  be  built  either  of  brick  or  of 
concrete.  Brick  is  more  commonly  used,  as  the  forms  necessary 
for  concrete  make  the  work  more  expensive  unless  they  can  be 
used  a  number  of  times.  The  walls  of  the  manhole  should  be  at 
least  8  inches  thick.  Greater  thicknesses  are  -used  in  treacherous 
soils  arid  for  deep  manholes,  or  to  exclude  moisture.  A  rough 
expression  for  the  thickness  of  the  walls  of  a  brick  manhole  more 

than  12  feet  deep  in  ordinary  firm  material  is  Z  =  ~+2,  in  which  t 

m 

99 


100 


APPURTENANCES 


is  the  thickness  in  inches  and  d  is  the  depth  in  feet.  The  thick- 
ness of  brick  walls  may  be  changed  every  5  to  10  feet  or  so.  Con- 
crete walls  may  be  built  thinner  than  brick  walls. 

The  bottoms  of  brick  manholes  are  frequently  made  of  con- 
crete as  shown  in  Fig.  27.  The  floor  slopes  towards  the  center 
and  is  constructed  so  that  the  sewage  flows  in  a  half  round  or 
U-shaped  channel  of  greater  capacity  than  the  tributary  sewers. 
The  sides  of  the  channel  should  be  high  enough  to  prevent  the 
overflow  of  sewage  onto  the  sloping  floor,  which  should  have  a 


Straight     Through 
Manhple. 


Manhole. 


Junction    Manhole. 
FIG.  27. — Baltimore  Standard  Manhole  Details. 

pitch  of  about  one  vertical  to  10  or  12  horizontal.  In  manholes 
where  two  or  more  sewers  join  at  approximately  the  same  level 
the  channels  in  the  bottom  should  join  with  smooth  easy  curves. 
Where  the  inlet  and  outlet  pipes  are  not  of  the  same  diameter 
the  tops  of  the  pipes  should  ordinarily  be  placed  at  the  same 
elevation  to  prevent  back  flow  in  the  smaller  pipes  when  the 
larger  pipes  are  flowing  full. 

The  dimensions  of  the  manhole  should  not  be  less  than  3  feet 
wide  by  4  feet  long  for  a  height  of  at  least  4  feet,  when  built  in 
the  form  of  an  ellipse,  or  4  feet  in  diameter  when  built  circular. 
No  standard  method  for  the  reduction  of  the  diameter  of  the  man- 
hole near  the  top  is  observed,  the  rate  being  more  or  less  dependent 


MANHOLES 


101 


on  the  depth  of  the  manhole.  The  use  of  sloping  sides  above  the 
frost  line  is  desirable  as  such  a  form  is  more  resistant  to  heaving 
by  frost  action. 

For  sewers  up  to  48  inches  in  diameter  the  manhole  is  usually 
centered  over  the  intersection  of  the  pipes  and  has  a  special  founda- 
tion. For  larger  sewers  the  manhole  walls  spring  from  the  walls 
of  the  sewer  as  shown  in  Fig.  28. 

In  the  case  of  a  decided  drop  in  the  elevation  of  a  sewer,  or  of 
a  tributary  sewer  appreciably  higher  than  an  outlet  in  any  man- 
hole, the  sewage  is  allowed  to  drop  vertically  at  the  manhole, 


Cross  Section.  Longitudinal  Section. 

Manhole     at    Omaha 


Well  Hole  at  St.  Paul. 

Fron,  Eng. Record. V»1.66.  p.J70^ 


FIG.  28. — Details  of  a  Manhole  and  a  Well  Hole. 

hence  the  name  drop  manhole.  The  Baltimore  standard  drop 
manhole  is  shown  in  Fig.  27.  A  well  hole  is  an  unusually  deep 
drop  manhole  in  which  the  force  of  the  vertical  drop  of  sewage 
is  broken  by  a  series  of  baffle  plates,  or  by  a  sump  at  the  bottom 
of  the  well  hole.  Fig.  28  shows  a  well  hole  at  St.  Paul,  Minn. 
The  use  of  drop  manholes  can  be  avoided  in  large  sewers  by  the 
construction  of  a  flight  of  steps  or  flight  sewer  as  shown  in  Fig.  29, 
which  allows  the  use  of  a  steep  grade  and  serves  to  break  the 
velocity  of  the  sewage. 

The  specifications  of  the  Sanitary  District  of  Chicago,  cover- 
ing the  construction  of  manhole  covers  and  frames  are: 

All  castings  shall  be  of  tough,  close  grained,  gray  iron, 
free  from  blow  holes,  shrinkage  and  cold  shuts,  and  sound, 
smooth,  clean  and  free  from  blisters  and  all  defects. 


102  APPURTENANCES 

All  castings  shall  be  made  accurately  to  dimensions  to 
be  furnished  and  shall  be  planed  where  marked  or  where 
otherwise  necessary  to  secure  perfectly  flat  and  true  sur- 
faces. Allowance  shall  be  made  in  the  patterns  so  that  the 
specified  thickness  shall  not  be  reduced. 

All  castings  shall  be  thoroughly  cleaned  and  painted 
before  rusting  begins  and  before  leaving  the  shop  with 
two  coats  of  high  grade  asphaltum  or  any  other  varnish 
that  the  Engineer  may  direct.  After  the  castings  have 
been  placed  in  a  satisfactory  manner,  all  foreign  adhering 
substances  shall  be  removed  and  the  castings  given  one 
additional  coat  of  asphaltum.  No  castings  shall  be 
accepted  the  weight  of  which  shall  be  less  than  that  due  to 
its  dimensions  by  more  than  5  per  cent. 


FIG.  29. — Flight  Sewer  at  Baltimore. 

Eng.  Record,  Vol.  59,  p.  161. 

The  weights  of  frames  and  covers  in  use  vary  from  200  to  600 
pounds,  the  weight  of  the  frame  being  about  5  times  that  of  the 
cover.  The  lightest  weights  are  used  where  no  traffic  other  than 
an  occasional  pedestrian  will  pass  over  the  manhole.  Frames 
and  covers  weighing  about  400  pounds  are  commonly  used  on 
residential  streets,  whereas  600  pound  frames  and  covers  are 
desirable  in  streets  on  which  the  traffic  is  heavy.  The  frames 
should  be  so  designed  that  the  pavement  will  rest  firmly  against 
it  and  wear  at  the  same  rate  as  the  surrounding  street  surface. 
Experience  has  shown  that  vertical  sides  should  be  used  for  the 
outside  of  the  frame  to  approach  this  condition,  and  that  the  frame 
should  not  be  less  than  8  inches  high.  The  cover  should  be  rough- 
ened in  some  desirable  pattern  as  shown  in  Fig.  30.  Smooth 
covers  become  dangerously  slippery.  Where  the  ventilation  of 


MANHOLES 


103 


the  sewers  is  not  satisfactory  the  manhole  covers  are  sometimes 
perforated.  This  is  undesirable  from  other  points  of  view  as  the 
rising  odors  and  vapors  are 
obnoxious  at  the  surface  and 
the  entering  dirt  and  water  are 
detrimental  to  the  operation 
of  the  sewer.  The  stealing 
and  destruction  of  manhole 
covers  and  the  unauthorized 
entering  of  sewers  has  occa- 
sionally required  the  locking 
of  the  covers  to  the  frame  when 
in  place.  The  locks  commonly 
used  consist  of  a  tumbler  which 
falls  into  place  when  the  man- 
hole is  closed,  and  which  can  be 
opened  only  by  a  special  wrench 

or    hook.      Adjustable    frames  ** 

are  sometimes  used  where  the  FIG.  30. — Baltimore  Standard  Manhole 
street  grade  is  settling,  or  may  Frame  and  Cover, 

be    raised    in    order    that    the 

elevation  of  the  top  of  the  cover  may  be  made  to  conform  to  that 
of  the  street  surface,  without  reconstructing  the  top  of  the  man- 
hole. One  type  of  adjustable  cover  is  shown  in  Fig.  31.  Man- 


-CornJbations 
>.AandB 
Covers. 

and  Bosses  tobeJemittedon 
[  andC  CovepSCoyer  C  to  be  used 

on  all  Sidewalks  and 
jj.  Crown  elsewhere  as  di reefed. 

x>-jj       ^Frame  and  Cover  asphalf1- 
" — \ed.Corruga- 


FIG.  31. — Adjustable  Manhole  Frame  and  Cover. 

hole  covers  should  be  so  marked  that  the  sanitary  sewer  can  be 
distinguished  from  the  storm  water  sewer,  and  both  from  the 
telephone  conduit,  etc. 

Iron  steps  are  set  into  the  walls  of  the  manhole  about  15 
inches  apart  vertically  to  allow  entrance  and  exit  to  and  from  the 
manhole.  Galvanized  iron  is  preferable  to  unprotected  metal  as 
the  action  of  rust  is  particularly  rapid  in  the  moist  air  of  the  sewer. 


104 


APPURTENANCES 


One  type  of  these  manhole  steps  is  shown  in  Fig.  27.  The  steps 
should  have  a  firm  grip  in  the  wall  as  a  loose  step  is  a  source  of 
danger. 

57.  Lampholes. — A  lamphole  is  an  opening  from  the  surface 
of  the  ground  into  a  sewer,  large  enough  to  permit  the  lowering 
of  a  lantern  into  the  sewer.     Lampholes  are  used  in  the  place  of 

manholes  to  permit  the  inspec- 
tion or  the  flushing  of  sewers, 
and  to  avoid  the  expense  of  a 
manhole.  They  are  located 
from  300  to  400  feet  from 
the  nearest  manhole  in  such 
a  manner  that  a  lamp  lowered 
in  the  lamp  hole  can  be  seen 
from  the  two  nearest  man- 
holes. 

Lampholes  should  be  con- 
structed of  8- to  12-inch  tile 
or  cast-iron  pipe.  The  lower 
section  should  be  a  cast  iron 
T  on  a  firm  foundation,  but 
if  constructed  of  tile  it  should 
be  reinforced  with  concrete 
to  take  up  the  weight  of  the 
shaft.  The  details  of  the 
Baltimore  standard  lamphole 
are  shown  in  Fig.  32. 
Lampholes  are  not  com- 
monly used  on  sewerage  sys- 
tems on  account  of  their 
lack  of  real  usefulness  and 
the  troubles  encountered  by 
breaking  of  the  pipe  below  the  shaft. 

58.  Street  Inlets. — A  street  inlet  is  an  opening  in  the  gutter 
through  which  storm  water  gains  access  to  the  sewer.     The  types 
used  in  different  cities  vary  widely.     Details  of  an  inlet  in  success- 
ful use  are  shown  in  Fig.  33.     The  figure  shows  also  a  common 
form  of  connection  to  the  sewer.     A  water-seal  trap  is  sometimes 
used  to  prevent  the  escape  of  odors  from  the  sewer.     Care  must 
be  taken  in  design  that  such  traps  do  not  freeze  in  winter  nor  dry 


FIG.  32. — Baltimore  Standard  Lamphole. 


STREET  INLETS 


105 


£_ 

?'5 

"   il 

* 

1 

Tk 

r* 

n] 

L_ 

_|j 

(7^ 

*n 

0 

i^r 

.i 

^ 

V 

[7- 

—  TT 

"c 

f-i* 

I  1 

*Q 

1* 

I  1 

(J 

F* 

—  n 
j  i 

1 

tz 

-dJ 

I 

—  —  ^ 

out  in  summer,  although  it  is  not  always  possible  to  prevent 
these  contingencies. 

The  important  features  to  be  observed  in  the  design  of  a  street 
inlet  are:  height  and  length  of  opening,  character  of  grating,  and 
location.  The  general  location  of  inlets  is  discussed  in  Chapter  V. 
The  clear  height  of  opening  commonly  used  is  from  5  to  6  inches, 
with  a  clear  length  of  24  to  30 
inches  or  longer.  Inlets  of  this 
size  have  given  satisfaction  on 
paved  streets  with  moderate  slopes, 
where  the  drainage  area  is  not 
greater  than  10,000  to  12,000 
square  feet  of  pavement.  W.  W. 
Horner  states:1 

The  St. Louis  type  of  inlet 
"  old  "  style  was  a  vertical 
opening  in  the  curb  8  inches 
high  and  4  feet  in  length 
with  a  horizontal  bar  mak- 
ing the  net  opening  about 
5  inches.  It  has  a  broad 
sill  extending  under  the 
sidewalk.  The  "  new  "  style 
inlet  is  4^  feet  long  with  a 
clear  opening  of  6  inches 
and  no  bar.  The  sill  is  done 
away  with  and  the  opening 
drops  down  directly  from 
the  curb.  Tests  were  made 
of  the  capacity  of  this  inlet 
on  pavements  on  different 
slopes  with  sumps  of  depths 
varying  from  0  to  6  inches  FIG.  33. — Details  of  an  Untrapped 
in  front  of  the  inlet,  extend-  Street  Inlet,  without  Catch-Basin, 
ing  out  3  feet  from  the  gutter, 

and  returning  to  the  elevation  of  the  gutter  at  a  slope  of  3 
inches  to  the  foot.  The  results  of  these  tests  are  shown  in 
Table  22.  The  capacity  of  the  inlet  is  expressed  as  the 
amount  of  water  entering  just  before  some  water  begins  to 
lap  past.  If  a  large  amount  of  water  is  allowed  to  flow  past 
much  more  water  will  enter  the  inlet  thus  furnishing  a 
factor  of  safety  for  large  storms.  It  was  noted  that  by 
beginning  the  rise  in  the  pavement  about  opposite  the 

1  Municipal  and  County  Engineering,  October,  1909. 


k— •- 26" ol 


106 


APPURTENANCES 


middle  of  the  inlet  the  capacity  of  the  inlet  was  increased 
from  20  to  50  per  cent. 

TABLE  22 

CAPACITIES  OF  ST.  Louis  STREET  INLETS 
From  tests  by  W.  W.  Horner.    Cubic  feet  per  second 


Slope  in  Per  Ct. 

0.42 

1.5 

2.85 

4.5 

Depth  of  Sump, 

Inches  

0.0 

2 

4 

6 

0. 

2 

4 

6 

0 

2 

4 

6 

0 

2 

4 

6 

Capacity,       old 

style 

1  27 

0  03 

0  25 

0  78 

1  49 

Capacity,     new 

style  

0.1 

0.5 

1.5 

2.5 

0.08 

0.4 

1.1 

2.1 

0.03 

0.28 

0.87 

1.62 

0.02 

0.15 

0.45 

1.0 

Gratings  with  horizontal  bars  will  admit  more  water  than 
gratings  with  vertical  bars,  but  they  will  also  admit  more  rubbish 
such  as  sticks,  papers,  leaves,  etc.,  which  serve  to  clog  the  sewers. 
Vertical  barred  gratings  and  gratings  in  the  bottom  of  the  gutter 
clog  more  quickly  than  other  types.  In  the  selection  of  the  type 
of  grating  to  be  used  a  decision  must  be  made  as  to  whether  it  is 
more  desirable  to  clean  the  sewer  or  catch-basin,  or  to  flood  the 
street  as  a  result  of  clogged  inlets.  Where  catch-basins  are  used 
or  the  sewers  are  large,  horizontal  bars  are  more  satisfactory. 
The  openings  between  bars  should  be  small  enough  to  prevent 
the  entrance  of  a  horse's  hoof  or  objects  of  sufficient  size  to  clog 
the  sewer.  Four  inches  in  the  clear  for  vertical  openings  and  6 
inches  for  horizontal  openings  are  reasonable  sizes. 

The  location  of  the  inlets  at  the  intersection  of  the  two  curb 
lines  at  a  corner  results  in  a  lower  first  cost  but  on  heavily  traveled 
streets  this  may  result  in  a  higher  maintenance  cost  than  for  other 
locations  because  of  the  concentration  of  traffic  at  street  corners, 
hammering  the  inlet  casting  out  of  shape  or  position.  Vehicles 
making  short  turns  will  tend  to  climb  the  curb  and  will  intensify 
the  wear  upon  the  inlet.  These  objections  can  be  overcome  by 
the  use  of  two  inlets  at  each  corner,  set  back  far  enough  from  the 
curb  intersection  to  avoid  interference  with  the  cross-walks. 
This  also  makes  it  possible  to  raise  the  cross-walks  without  the 
use  of  gutters  under  them. 

The  size  of  the  pipe  from  the  inlet  to  the  catch-basin  or  sewer 
should  be  large  enough  to  care  for  all  of  the  water  which  may  enter 


CATCH-BASINS 


107 


the  inlet.  As  the  capacity  of  the  inlet  is  seldom  known  with  accu- 
racy and  the  capacity  of  the  pipe  is  difficult  of  determination,  it 
has  become  customary  to  use  a  10-inch  or  a  12-inch  connecting 
pipe  for  each  ordinary  independent  inlet. 

59.  Catch-basins. — Catch-basins  are  used  to  interrupt  the 
velocity  of  sewage  before  entering  the  sewer,  causing  the  deposi- 
tion of  suspended  grit  and  sludge  and  the  detention  of  floating 
rubbish  which  might  enter  and  clog  the  sewer.  A  separate  catch- 


Street 

Inlet 


FIG.  34. — CATCH-BASIN. 

Outlets  are  not  always  trapped. 

basin  may  be  used  for  each  inlet,  or  to  save  expense,  the  pipes  from 
several  inlets  may  discharge  into  one  catch-basin. 

The  types  in  successful  use  are  extremely  varied,  but  the  dis- 
tinguishing feature  of  all  is  an  outlet  located  above  the  floor  of 
the  basin.  A  common  form  of  catch-basin  is  shown  in  Fig.  34. 
It  is  constructed  similar  to  a  manhole  with  a  diameter  of  about 
4  or  4^  feet  and  a  depth  of  retained  water  from  3  to  4  feet.  Catch- 
basins  of  this  size  will  care  for  the  sewage  from  the  inlets  at  the 
four  corners  of  a  street  intersection,  each  draining  a  city  block. 


108  APPURTENANCES 

In  unusual  situations  it  may  be  necessary  to  install  a  larger  basin, 
but  too  large  a  catch-basin  is  less  desirable  than  one  which  is  too 
small,  as  the  former  stinks  and  the  latter  is  useless.  Traps  are 
sometimes  used  to  prevent  the  escape  of  odors  from  the  sewer 
into  the  street,  but  odors  are  often  created  in  the  catch-basins 
themselves.  Some  engineers  arrange  the  trap  so  that  it  can  be 
opened  for  observation  down  the  sewer  as  in  Fig.  34,  thus  com- 
bining the  advantages  of  a  manhole  with  the  catch-basin. 

The  use  of  catch-basins  is  objectionable  because:  they  furnish 
a  breeding  place  for  mosquitoes  and  other  flying  insects;  the 
septic  action  in  them  produces  offensive  odors;  if  on  a  combined 
sewer  they  permit  the  escape  of  offensive  odors  in  dry  weather 
when  the  water  seal  in  the  trap  has  evaporated;  and  the  freezing 
of  the  water  seal  in  the  trap  prevents  the  entrance  of  water  to 
the  sewer.  The  sole  advantage  lies  in  the  prevention  of  the 
clogging  of  the  sewers,  but  this  may  be  sufficient  to  overbalance 
all  of  the  disadvantages.  In  general  catch-basins  should  be 
provided  on  paved  streets  which  are  cleaned  by  flushing  the 
material  into  the  sewers,  or  where  the  drainage  is  from  an  unim- 
proved or  macadamized  street,  sandy  country,  or  into  sewers  in 
which  the  velocity  of  flow  is  less  than  2  feet  per  second. 

60.  Grease  Traps. — The  presence  of  grease  in  sewers  results 
in  the  formation  of  incrustations  which  are  difficult  to  remove 

and  which  cause  a  material  loss  in 
the  capacity  of  the  sewer.  The 
presence  of  oil  and  gasoline  has  re- 
sulted in  violent  and  destructive  ex- 
plosions as  is  described  in  Chapter 
XII.  A  type  of  grease  trap 'used  on 
the  drains  from  hotels,  restaurants, 
or  other  large  grease  producing  indus- 

FIG.  35.-Diagrammatic    Sec-  tries.  is   shown  in   Yi%'  35'      The  traP 
tion  through  a  Grease  Trap.      ig    similar    to    a    catch-basin    except 

that  it   is   too    small   for   a   man    to 

enter,  and  the  outlet  is  necessarily  trapped  in  order  to  pre- 
vent the  escape  of  grease.  The  details  of  a  gasoline  and  oil 
separator  approved  by  the  New  York  City  Fire  Department  are 
shown  in  Fig.  36.1 

1  "Cleaning  and  Flushing  Sewers. "  Journal  of  the  Association  of  Engineer- 
ing Societies,  Vol.  33,  1904,  p.  212. 


FLUSH-TANKS 


109 


fW. 


4  Clean-out 


FIG.  36. — Gasoline  and  Oil  Separator. 


61.  Flush-Tanks. — These  are  devices  to  hold  water  used  in 
flushing  sewers.  They  are  required  only  on  sanitary  and  com- 
bined sewers.  Their  use  tends  to  prevent  the  clogging  of  sewers 
laid  on  flat  grades  and  permits 
flatter  grades  in  construction 
than  could  otherwise  be  adopt- 
ed. Flush-tanks  may  be  oper- 
ated either  by  hand  or  auto- 
matically. Automatic  operation 
is  more  common  than  hand 
operation.  The  hand-operated 
tanks  are  similar  to  manholes 
so  arranged  that  the  inlet  and 
outlet  sewers  can  be  plugged 
while  the  manhole  or  tank  is 
being  filled  with  water  either 
from  a  hose  or  a  special  service 
connection.  When  sufficient 
water  has  been  accumulated 
the  outlet  is  opened  and 

the  sewer  is  flushed  by  the  rush  of  water.  A  sluice  gate,  flap 
valve,  or  a  specially  fitted  board  is  sufficient  to  fit  over  the  mouth 
of  the  inlet  and  outlet  during  the  filling  of  the  tank.  Such  an 
arrangement  has  the  advantage  of  being  cheap,  simple,  and 
satisfactory,  though  somewhat  crude.  In  some  cases  water  is 
run  into  the  tank  at  the  same  rate  that  it  is  discharged  through 
the  open  outlet,  maintaining  a  depth  of  4  or  5  feet  in  the  tank 
until  the  water  passing  the  manhole  below  runs  clean.  The 
volume  of  water  required  by  this  method  is  large.  Flushing 
water  under  a  relatively  high  head  is  sometimes  obtained  by  the 
use  of  tank  wagons  which  are  quickly  emptied  into  the  sewer 
through  a  canvas  pipe  dropped  down  a  manhole.  In  all  such 
cases  if  not  well  constructed  the  manhole  is  subject  to  caving  due 
to  the  rush  of  water  around  the  outlet.  Precautions  should  be 
taken  to  minimize  this  danger  by  limiting  the  depth  of  water 
which  may  be  accumulated.  This  can  be  done  by  constructing 
an  overflow  at  a  height  of  4  or  5  feet  above  the  bottom  of  the  man- 
hole, discharging  into  the  sewer  through  an  outside  drain. 

Automatic  flush-tanks  are  constructed  similar  to  a  manhole, 
but  special  care  should  be  taken  to  make  them  water-tight.     The 


110 


APPURTENANCES 


Regu/a- 


apparatus  for  providing  the  automatic  discharge  may  operate 
either  with  or  without  moving  parts,  the  latter  being  preferable 
as  they  require  less  attention  and  are  not  so  liable  to  get  out  of 
order.  An  automatic  flush-tank  of  the  Miller  type  is  shown  in 
Fig.  37.  It  is  a  patented  device  manufactured  by  the  Pacific 

Flush  Tank  Company.  The 
small  pipe  at  the  left  is  a 
service  connection  to  the  water 
main.  Water  is  allowed  to  flow 
continuously  into  the  tank  at 
such  a  rate  as  to  fill  it  in  the 
required  interval  between  dis- 
charges. The  tanks  are  dis- 
charged as  nearly  once  a  day 
as  it  is  practicable  to  regulate 
them.  The  rate  of  flow  into 
the  tank  is  determined  by  trial 
and  varies  to  some  extent  with 
the  water  pressure.  The  regu- 
lator shown  in  the  figure  is 
desirable  as  the  continuous  flow 
through  the  ordinary  cock  soon  wears  it  away.  Some  waters 
will  cause  deposits  to  form  in  the  small  passages  of  the  cocks  or 
regulators,  thus  cutting  off  the  flow. 

The  tank  operates  as  follows:  when  the  water  rising  in  the 
tank  reaches  the  bottom  of  the  bell,  air  is  trapped  in  the  bell  and 
prevented  from  escaping  through  the  main  trap  by  the  water  at  A . 
As  the  water  continues  to  rise  in  the  tank  the  air  in  the  bell  is 
compressed,  the  water  level  at  A  is  driven  down  and  water  trickles 
from  the  siphon  at  C.  The  height  of  the  water  in  the  tank  above 
the  level  of  the  water  in  the  bell  is  equal  at  all  times  to  the  height 
of  C  above  the  lowering  position  of  A.  When  A  reaches  the 
position  of  B  a  small  amount  of  air  is  released  through  the  short 
leg  of  the  trap  and  a  corresponding  volume  of  water  enters  the 
bell.  The  head  of  water  above  the  bell  then  becomes  greater 
than  the  head  of  water  in  the  short  leg  of  the  trap,  which  results 
in  the  discharge  of  all  of  the  air  in  the  long  leg  of  the  trap  and 
the  rapid  discharge  of  the  water  in  the  tank  through  the  siphon. 
The  discharge  is  continued  until  the  siphonic  action  is  broken  by 
the  admission  of  air  when  the  water  level  in  the  tank  is  lowered 


FIG.  37. — Automatic  Flush-Tank. 

Pacific  Flush  Tank  Co. 


FLUSH-TANKS 


111 


to  the  bottom  of  the  bell.  The  size  of  the  siphons  is  fixed  by  the 
diameter  of  the  leg  of  the  siphon.  Table  23  shows  the  capacity 
and  size  of  sewers  for  which  the  different  sizes  of  siphons  are 
recommended  by  the  manufacturers.1 

TABLE  23 

SIZES  OF  SIPHONS  TO  BE  USED  WITH  AUTOMATIC  FLUSH-TANKS 


Diameter 

r»f 

Diameter 
of  Tank 

Total 
Discharge 

Average 
Rate 

Diameter 
of 

Height  of 
the 

Siphon 

at  the 
Discharge 

for  One 
Flush 

of 
Discharge 

Sewer 

Discharge 
Line  above 

in 
Inches 

Line 
in  Feet 

in 
Gallons 

in 
Sec  .-ft. 

Inches 

the  Edge  of 
the  Bell 

4 

3 

60 

0.35 

4  to    6 

1  ft.    2  in. 

5 

3 

100 

0.73 

6  to    8 

1  ft.  11  in. 

6 

4 

240 

1.0§ 

8  to  10 

2  ft.    6  in. 

8 

4 

280 

2.12 

12  to  15 

2  ft.  11  in. 

When  flush-tanks  are  placed  at  the  upper  end  of  laterals 
provision  should  be  made  for  inspecting  and  cleaning  the  sewer 
by  the  construction  of  a  separate  manhole,  or  by  combining  the 
features  of  a  manhole  and  a  flush-tank  in  the  same  structure. 
Such  a  combination  is  shown  in  Fig.  38  from  a  design  by  Alex- 
ander Potter. 

Except  under  unusual  conditions  flush-tanks  are  used  only  on 
separate  sewers.  They  should  be  placed  at  the  upper  end  of 
laterals  in  which  the  velocity  of  flow  when  full  is  less  than  2  to 
4  feet  per  second.  The  capacity  of  the  tank  or  the  volume  of  the 
dose  is  dependent  on  the  diameter  and  slope  of  the  sewer.  The 
most  effective  flush  is  obtained  by  a  volume  of  water  traveling 
at  a  high  velocity  and  completely  filling  the  sewer.  A  .large 
volume  allowed  to  run  slowly  through  the  sewer  will  have  but 
little  if  any  flushing  action.  Data  on  the  quantity  of  flushing 
water  needed  are  given  in  Table  24. 2  As  the  result  of  a  series  of 
experiments  conducted  by  Prof.  H.  N.  Ogden  on  the  flushing  of 

1  Notes  on  the  Design  and  Principles  of  Sewage  Siphons,  Eng.  News-Record, 
Vol.85,  1920,  p.  1041. 

2  From  A.  E.  Phillips,  Trans.  Am.  Society  of  Municipal  Improvements, 
1898,  p.  70. 


112 


APPURTENANCES 


v////n\  i  i    i     i  i  i  i-  WS//A 

?W\  '    '    '  '  '  '  W$fe:m3h  Wafer  line 

• -4JP, '  J  , '  ,  '  ,  '  ,  '  ,  ' , '  M^-^f,. 

fli.  1. 1  j    '     i  i    I    I  jj  ItejftrV                  — ' 


Podding  Trough- 

Floor 
RemovableCap 


Sectional 
Plan. 


FIG.  38. — Automatic  Flush-Tank  and  Manhole. 

Miller-Potter  Design.     Pacific  Flush  Tank  Co. 

TABLE  24 
GALLONS  OF  WATER  NEEDED  FOR  FLUSHING  SEWERS 


Diameter  of  Sewer  in  Inches 

•slope 

8 

10 

12 

0.005 

80 

90 

100 

.0075 

55 

65 

80 

.01 

45 

55 

70 

.02 

20 

30 

35 

.03 

15 

20 

24 

SIPHONS  113 

sewers,1  the  conclusion  was  reached  that  the  effect  of  a  flush  of 
about  300  gallons  in  an  8-inch  sewer  on  a  grade  less  than  1  per 
cent  would  not  be  effective  beyond  800  to  1,000  feet,  but  that  on 
steeper  grades  much  smaller  quantities  of  water  would  produce 
equally  good  results. 

Engineers  do  not  agree  upon  the  advisability  of  the  use  of 
automatic  flush-tanks,  some  believing  that  they  are  a  needless 
expense  that  can  be  avoided  by  hand  flushing,  and  others  feeling 
that  a  flush-tank  should  be  placed  at  the  upper  end  of  every  lateral. 
These  diverse  opinions  are  the  result  of  different  experiences  in 
different  cities. 

62.  Siphons. — There  are  two  forms  of  siphons  used  in  sewerage 
practice,  a  true  siphon  and  an  inverted  siphon.  A  true  siphon 
is  a  bent  tube  through  which  liquid  will  flow  at  a  pressure  less 
than  atmospheric,  first  upwards  and  then  downwards,  entering 
and  leaving  at  atmospheric  pressure.  An  inverted  siphon  is  a  bent 
tube  through  which  liquid  will  flow  at  a  pressure  greater  than 
atmospheric  first  downwards  and  then  upwards,  entering  and 
leaving  at  atmospheric  pressure. 

In  sewerage  practice  the  word  siphon  refers  to  an  inverted 
siphon  unless  otherwise  qualified.  Siphons,  both  true  and 
inverted,  are  used  in  sewerage  systems  to  pass  above  or  below 
obstacles.  True  siphons  are  seldom  used  as  they  must  be  kept 
constantly  filled  with  liquid.2  Accumulated  gas  must  be  removed 
in  order  to  prevent  the  breaking  of  the  siphon  which  results  in  the 
cessation  of  flow.  By  the  breaking  of  a  true  siphon  is  meant  the 
stoppage  of  siphonic  action  due  to  the  accumulation  of  air  or  gas 
at  the  peak  of  the  siphon.  Since  the  rate  of  flow  of  sewage  fluc- 
tuates widely  it  is  extremely  difficult  to  control  the  flow  so  that  a 
true  siphon  may  be  completely  filled  with  liquid  at  all  times. 

In  the  design  of  inverted  siphons  care  must  be  taken  to  pre- 
vent sedimentation,  and  to  permit  inspection  and  cleaning. 
Sedimentation  is  prevented  by  maintaining  a  velocity  greater 
than  a  fixed  minimum,  usually  taken  at  about  2  feet  per  second. 
This  minimum  is  attained  by  providing  a  number  of  channels. 
The  smallest  channel  is  designed  to  convey  the  least  expected 
flow  at  the  minimum  velocity.  Each  of  the  other  channels  is 
made  as  small  as  possible,  within  the  limits  of  economy  and  sim- 

1  Trans.  Am.  Society  of  Civil  Engineers,  Vol.  15,  1886. 

2  True  Siphon  at  East  Providence,  Eng.  News-Record,  Vol.  85,  1920,  p.  862. 


114 


APPURTENANCES 


plicity,  in  order  that  the  minimum  velocity  shall  be  exceeded 
quickly  after  flow  has  commenced  in  them.  The  last  channel  or 
channels  to  be  filled  are  made  somewhat  larger,  because  the 
sewage  conveyed  in  them  contains  less  settleable  matter  than  is 
contained  in  the  more  concentrated  dry  weather  flow.  The  type 
of  siphon  used  in  New  York  to  pass  under  the  subway  is  shown  in 
Fig.  39.  Note  should  be  taken  of  the  clean-out  manhole  provided 
on  the  14-inch  pipe.  The  other  pipes  are  large  enough  for  a  man 
to  enter  and  clean. 


Old  „„  4'-6"Ct'rcularl?6tnf.Concreie 

4-/Ox3L6"         P  M-fyP™  (Storm)Pipe^ 

Sewer 


Flout 


-Cleanout  Chamber 
Cleanout  Manhole 


Section  A-A. 


2t4-'-6"(Storm)  Pipes-' 

Longitudinal   Section , 


/4"CIP>'pe(DryWea- 

pPh 

Section  B-B. 


FIG.  39. — Sewer  Siphon  under  New  York  Subway. 

Eng.  News  Vol.  76,  p.  443. 

The  computations  involved  in  the  design  of  a  siphon  are 
illustrated  in  the  following  example,  in  which  it  is  desired  to  con- 
struct a  siphon  to  pass  under  the  railway  cut  shown  in  Fig.  40. 
The  first  step  is  to  determine  the  limiting  diameter  and  slope  of 
the  smallest  pipe  in  the  siphon.  One-sixth  of  the  capacity  of  the 
6-foot  approach  sewer  or  19  cubic  feet  per  second  will  be  assumed 
as  the  minimum  flow.  The  diameter  of  the  pipe  necessary  to 
carry  19  cubic  feet  per  second  at  a  velocity  of  2  feet  per  second  is 
42  inches.  The  hydraulic  gradient  should  have  a  slope  of  0.0005 
if  the  material  used  has  a  roughness  coefficient  of  .015.  This  is 
ths  minimum  permissible  slope  of  the  siphon.  The  selection  of  a 
steeper  slope  will  necessitate  the  laying  of  the  sewer  at  a  greater 
depth,  and  will  permit  the  use  of  smaller  pipes  for  the  siphon. 


SIPHONS 


115 


The  selection  of  the  exact  slope  must  then  be  based  on  judgment 
with  the  minimum  limitation  above  placed.  The  slope  will  be 
arbitrarily  selected  as  0.001,  the  same  as  that  of  the  approach 
sewer.  The  diameter  of  the  dry  weather  pipe  will  therefore  be 
36  inches,  with  a  capacity  of  18  second-feet,  which  is  approximately 
the  assumed  dry-weather  flow.  The  velocity  of  flow  will  be 
2.5  feet  per  second.  The  length  of  flow  along  the  siphon  is  150  feet. 
The  next  step  should  be  the  determination  of  the  elevation  at 
the  lower  end  of  the  36-inch  pipe.  This  is  done  by  multiplying 
1 


Vertical  Cross  Secfion 


60    Sewer 


42.    Sewer 


36  "  Sewer 


Plan  under  Retaining  Wall 
FIG.  40. — Diagram  for  the  Design  of  an  Inverted  Siphon. 

the  assumed  grade  by  the  equivalent  length  of  straight  pipe,  and 
subtracting  the  product  from  the  elevation  at  the  upper  end. 
The  length  of  straight  pipe  which  will  give  the  same  loss  of  head 
as  the  siphon  is  called  the  equivalent  pipe.  It  is  determined  as 
follows : 

First,  determine  the  head  loss  at  entrance.  This  will  vary 
between  nothing  and  one  velocity  head,  dependent  on  the  arrange- 
ment at  the  entrance.1  The  length  of  straight  pipe  which  will 

1  "The  Effect  of  Mouthpieces  on  The  Flow  of  Water  Through  a  Sub- 
merged Short  Pipe,"  by  F.  B.  Seely.  Bulletin  No.  96,  1917,  of  the  Eng'g. 
Experiment  Station  of  the  University  of  Illinois. 


116  APPURTENANCES 

give  this  same  loss  can  be  computed  from  the  expression  1=-^, 

using  for  S  the  assumed  slope  of  the  hydraulic  gradient. 

Second,  determine  the  head  loss  due  to  the  bends.  This  is 
determined  from  the  expression 

A=£^ 

d2g 

in  which  /i  =  the  head  loss  in  the  bend; 
Z  =  the  length  of  the  bend; 
d  =  the  diameter  of  the  pipe ; 
v  =  the  average  velocity  of  flow ; 
gr  =  the  acceleration  due  to  gravity; 
/=a  factor  dependent  on  the  radius  (R)  of  the  bend 
and  d. 

The  relation  between  /,  R,  and  d,  for  90°  bends  is  shown  as 

follows:1 

R/d        24  16  10  6  4  2.4 

/  0.036        0.037        0.047        0.060      0.062      0.072 

After  the  head  loss  has  been  determined,  the  equivalent  length  of 
straight  pipe  is  determined  as  above. 

Third.  The  equivalent  length  of  pipe  will  be  the  sum  of  the 
actual  length  of  pipe  and  the  equivalent  lengths  as  found  above. 

In  the  problem  in  hand  the  head  lost  at  the  entrance  will  be 
assumed  as  one-third  of  a  velocity  head,  or  0.0324  foot.  With 
the  assumed  slope  of  0.001  this  is  equivalent  to  32  feet  of  pipe. 
The  radius  of  the  bend  is  about  20  feet  and  the  length  for  a  45° 
central  angle  is  about  16  feet.  The  head  loss  for  this  angle  will 
probably  be  a  little  more  than  one-half  that  for  a  90°  angle.  The 

V2 
expression  will  therefore  be  taken  as  about  0.2  5—  and    for  two 

bends  is  equivalent  to  about  40  feet  of  pipe.  The  equivalent 
length  of  pipe  is  therefore  150+32+40  =  222  feet.  The  elevation 
at  the  lower  end  should  therefore  be :  the  elevation  at  the  upper 
end,  92.07-222X.001  =  91.85. 

The   diameters   of  the   remaining   pipes   in   the   siphon   are 

determined  so  that  the  sewage  in  the  approach  sewer  is  backed 

up  as  little  as  is  consistent  with  good  judgment  before  each  pipe 

comes  into  action.     This  is  done  satisfactorily  by  a  method  of 

1  Trans.  Am.  Society  of  Civil  Engineers,  Vol.  49,  1902. 


REGULATORS  117 

cut  and  try.  Let  it  be  assumed  that  the  siphon  will  be  composed 
of  three  pipes:  the  dry-weather  pipe  taking  18  second-feet,  the 
second  pipe  taking  28  second-feet,  and  the  third  pipe  taking  the 
remaining  70  second-feet.  The  diameters  of  the  two  larger  pipes 
on  the  assumed  slope  of  0.001  will  therefore  be  42  inches  and  60 
inches  respectively.  Other  combinations  might  be  used  which 
would  be  equally  satisfactory.  There  are  many  methods  by  which 
the  sewage  can  be  diverted  into  the  different  channels  of  the 
siphon.  For  example,  the  openings  into  the  different  pipes  may 
be  placed  at  the  same  elevation,  and  the  sewage  allowed  to  enter 
them  in  turn  through  automatically  or  hand-controlled  gates,  or 
in  another  method  of  control  the  openings  may  be  placed  at  such 
elevations  that  when  the  capacity  of  one  pipe  has  been  exceeded 
the  sewage  will  flow  into  the  next  largest  pipe  as  shown  in  Fig.  40. 
The  outlets  from  the  different  pipes  are  ordinarily  placed  at  the 
same  elevation,  thus  leaving  each  pipe  standing  full  of  sewage. 
Stop  planks  should  be  provided  at  the  outlet  in  order  that  the 
pipes  may  be  pumped  out  for  cleaning.  The  objection  to  this 
arrangement  is  that  the  larger  pipes  may  operate  at  a  velocity 
less  than  2  feet  per  second,  and  they  will  be  standing  full  of 
sewage  which  might  become  septic.  However,  as  they  will  take 
nothing  but  the  storm  flow  near  the  top  of  the  sewer  no  difficulty 
should  be  encountered  from  sedimentation  in  them,  and  all  are 
large  enough  for  a  man  to  enter  for  inspection  or  cleaning. 

63.  Regulators. — Regulators  are  commonly  used  to  divert  the 
direction  of  flow  of  sewage  in  order  to  prevent  the  overcharging 
of  a  sewer  or  to  regulate  the  flow  to  a  treatment  plant.  Sewer 
regulators  are  of  two  types,  those  with  moving  parts  and  those 
without  moving  parts.  An  example  of  the  moving  part  type  is 
shown  in  Fig.  41.  In  this  type  as  the  sewage  rises  the  float  closes 
the  gate  to  the  inlet  sewer,  thus  preventing  the  entrance  of  sewage 
under  head  from  the  larger  sewer.  There  are  many  variations  in 
the  details  of  float-controlled  regulators,  but  the  principle  of  opera- 
tion is  similar  in  all.  These  regulators  can  be  adjusted  to  fix  the 
maximum  rate  of  flow  to  a  relief  channel  or  sewage  treatment 
plant,  or  during  times  of  storm  to  cut  off  the  outlet  to  the  dry- 
weather  channel.  Another  form  of  the  moving  part  type  is  shown 
in  Fig.  42. 1  It  has  been  used  extensively  by  the  Milwaukee 

1  Described  by  W.  L.  Stevenson  before  the  Boston  Society  of  Civil  Engi- 
neers in  1916 . 


118 


APPURTENANCES 


Sewerage  Commission.  In  its  operation  the  dry-weather  flow  is 
diverted  by  the  dam  into  the  intercepter.  It  passes  under  the 
movable  gate  on  its  way  to  the  treatment  plant.  As  the  flow 

increases  the  dam  is 
overtopped  and  flood 
waters  are  discharged 
down  the  storm  chan- 
nel. The  movable  gate 
is  hung  on  a  pivot 
placed  below  center. 
As  the  water  rises  in 
the  intercepter,  the 
•Copper  Float  pressure  against  the 
upper  portion  of  the 
gate  becomes  greater 
than  that  against  the 
FIG.  41. — Coffin  Sewer  Regulator.  lower  portion,  and 

the  gate  closes.      An 

opening  is  left  at  the  bottom  to  allow  an  amount  of  sewage  equal 
to  the  dry-weather  flow  to  escape  beneath  the  gate  to  prevent 
clogging  or  sedimentation  in  the  intercepter  channel. 

Objections  to  all  moving  part  regulators  are  their  need  of 
attention  and  liability  to  become  clogged. 


Weight 


Direct/on 
of  Flow 


-Relief  Outlet 

-Dam 


Regulator 

Gate-. 


-  StormSewer- 
Longitudinal   Section 


-  Dry  Weather-  -Storm  Flow- 

Transverse  Sections  through  Regulator. 


FIG.  42. — Moving  Part  Regulator  without  Float. 

The  overflow  weir  and  the  leaping  weir  have  no  moving  parts 
and  are  used  for  the  regulation  of  the  flow  in  sewers.  A  leaping 
weir  is  formed  by  a  gap  in  the  invert  of  a  sewer  through  which 
the  dry-weather  flow  will  fall  and  over  which  a  portion  or  all  of 
the  storm  flow  will  leap.  One  form  of  leaping  weir  is  shown  in 
Fig.  43.  An  overflow  weir  is  formed  by  an  opening  in  the  side  of 
a  sewer  high  enough  to  permit  the  discharge  of  excess  flow  into  a 
relief  channel.  A  weir  at  San  Francisco  is  shown  in  Fig.  44.  A 
series  of  tests  were  run  on  leaping  weirs  and  overflow  weirs  in  the 
hydraulic  laboratory  of  the  University  of  Illinois.  The  type  of 


REGULATORS 


119 


leaping  weir  tested  was  formed  by  the  smooth  spigot  end  of  a  stand- 
ard vitrified  sewer  pipe.     The  overflow  weirs  were  formed  by  a 


Not  more 
than 
3ft  —  -> 


J-IOorIZ        Iron  Grating.^ 


-Brick 


Cast  Iron 
Grating 


~7 

Section  of  Inlet. 
Iron  Casting  to  here 
\  ,  Slot  Detail  not  to  Scab 


/ 

Longitudinal 
Section., 

^ 

; 

! 

'££[ 

T|l 

s  '•  ^s! 

3-SS 

r 

•  «S  i 

*— 

2" 

Plan  of  Inlet. 
FIG.  43. — Leaping  Weir  at  Danville,  Illinois. 


x  - .  Section  through  Hoof  Showing 

Reinforcement  under  Manhole. 
Sectional  Plan. 


Section  A-A.  Section  B-B.  Section  C-G. 

FIG.  44. — Overflow  Weir  at  San  Francisco. 
Eng.  News,  Vol.  73,  p.  307. 

steel  knife  edge  in  the  side  of  the  pipe  parallel  to  its  axis  as  shown 
in  Fig.  45.  Tests  were  made  in  18-inch  and  24-inch  pipes  on  various 
slopes  from  0.018  to  0.005,  for  both  leaping  weirs  and  overflow 


120 


APPURTENANCES 


weirs.  The  overflow  weirs  were  varied  in  length  from  16  inches 
to  42  inches  and  were  placed  at  various  heights  from  25  per  cent 
to  50  per  cent  of  the  diameter  above  the  invert  of  the  sewer.  As 
the  result  of  the  observations  the  following  formulas  were 
developed.  For  the  leaping  weir  the  expressions  for  the  coordi- 
nates of  the  curve  of  the  surfaces  of  the  falling  stream,  are  : 

For  the  outside  surface  x  = 
For  the  inside  surface  x  =  Q 


FIG.  45. — Overflow  Weir  in  Action.' 

Shadow  of  steel  knife  edge  which  forms  the  lip  of  the  weir   can  be  seen  through  the 
falling  sewage. 

in  which  x  and  y  are  the  coordinates.  The  origin  is  in  the  upper 
surface  of  the  stream  vertically  above  the  end  of  the  invert  of  the 
pipe.  The  ordinate  y  is  measured  vertically  downwards.  V  is 
the  velocity  of  approach  in  feet  per  second.  These  expressions 
are  applicable  to  any  diameter  of  sewer  up  to  10  or  15  feet.  They 
should  not  be  used  for  depths  of  flow  greater  than  about  14  inches; 
nor  for  slopes  of  more  than  25  per  1,000;  nor  for  velocities  less 
than  1  foot  per  second  nor  more  than  10  feet  per  second.  The 
expression  for  the  ordinate  of  the  inside  curve  is  not  good  for  less 


JUNCTIONS  121 

than  6  inches  nor  more  than  5  feet.  The  expression  for  the  ordi- 
nate  of  the  outside  curve  is  limited  to  values  between  the  origin 
and  5  feet  below  it. 

The  expression  for  the  length  of  an  overflow  weir  of  the  type 
shown  in  Fig.  45,  necessary  to  discharge  a  given  quantity,  is  in  the 
form, 


in  which    I  =the  length  of  the  weir  in  feet; 

V  =the  velocity  of  approach  in  feet  per  second; 
d  =the  diameter  of  the  pipe  in  feet; 
hi  =  the  head  of  water  on  the  upper  end  of  the  weir; 
ti2  =  the  head  of  water  on  the  lower  end  of  the  weir. 

In  the  design  of  an  overflow  weir  by  this  formula  the  height  of  the 
weir  above  the  invert  of  the  sewer  and  the  flow  over  the  weir 
should  be  determined  arbitrarily.  The  height  should  be  sub- 
tracted from  the  computed  depth  of  water  above  the  weir  to 
determine  the  value  of  hi.  The  difference  between  the  flow  over 
the  weir  and  the  flow  above  the  weir  will  represent  the  rate  of 
flow  in  the  sewer  below  the  weir.  The  value  of  h^  can  then  be 
computed  as  the  difference  in  the  depth  of  flow  below  the  weir 
and  the  height  of  the  weir  above  the  invert.  The  value  of  V  is 
computed  from  Kutter's  formula.  The  length  of  the  weir  is 
determined  by  substituting  these  values  in  the  formula. 

64.  Junctions.  —  At  the  junction  of  two  or  more  sewers  the 
elevation  of  the  inverts  should  be  such  that  the  normal  flow  lines 
are  at  the  same  elevation  in  all  sewers.  The  sewers  should 
approach  the  junction  on  a  steep  grade  to  prevent  sewage  backing 
up  in  one  when  the  other  is  flowing  full.  The  velocity  of  flow  at 
the  junction  should  not  be  decreased  and  turbulence  should  be 
avoided  in  order  to  prevent  sedimentation  and  loss  of  head. 
The  junction  should  be  effected  on  smooth  easy  curves  with  radii 
at  least  five  times  the  diameter  of  the  sewer  where  possible. 
Curves  with  short  radii  cause  backing  up  of  sewage  thus  reducing 
the  capacity  of  the  sewers. 

The  terms  bellmouth  or  trumpet  arch  are  sometimes  applied 
to  the  junction  of  sewers  large  enough  to  be  entered  by  a  man. 
In  small  sewers  the  Y  branches  and  special  junctions  are  manu- 
factured so  that  the  center  lines  of  the  pipes  intersect,  and  the 


122  APPURTENANCES 

junctions  of  mains  and  laterals  are  made  in  manholes.  In  the 
construction  of  a  bellmouth  the  arch  is  carried  over  all  the  sewers. 
A  manhole  should  be  constructed  at  these  junctions  as  clogging 
frequently  occurs  there,  due  to  swirling  and  back  eddies  which 
cannot  be  avoided. 

65.  Outlets. — The  outlets  to  a  sewerage  system  discharging 
into  a  swiftly  running  stream  must  be  protected  against  wash 
and  floating  debris.  In  a  stream  or  other  body  of  water  subject 
to  wide  variations  in  elevation  the  backing  up  of  the  sewage  during 
high  water  should  be  avoided.  Where  tidal  flats  or  low  ground 
about  the  outlet  may  be  alternately  submerged  and  uncovered 
the  discharge  should  always  be  into  swiftly  running  water.  In 
quiescent  bodies  of  water  such  as  lakes  and  harbors,  and  in  rivers 
where  the  dilution  is  low,  and  in  many  other  cases,  the  sewer 
outlet  should  be  submerged. 

Outlets  are  protected  against  wash  and  the  impact  of  debris 
by  the  construction  of  deep  foundations  and  heavy  protecting 
walls.  Although  the  construction  of  an  outlet  in  a  slow  current 
or  a  back  eddy  would  avoid  danger  from  wash  and  debris,  the 
discharge  of  the  sewage  into  the  most  rapid  current  possible  aids 
in  the  prevention  of  a  local  nuisance.  A  row  of  batter  piles  on 
the  upstream  or  exposed  side  of  the  sewer  is  desirable,  or  it  may 
be  necessary  to  construct  a  break-water  to  prevent  the  wash  of 
the  current  from  dislodging  the  pipe.  These  break-waters  are 

low  dams  of  wood  or  broken 
stone,  more  or  less  loosely 
thrown  together.  The  back- 
ing up  of  water  into  the 
sewer  can  be  prevented  by 
constructing  the  sewer  above 
PIG.  46. — Tide  Gate.  the  outlet  on  a  steep  grade. 

Where   this    is    not    possible 

the  use  of  tide  gates  will  be  helpful.  A  tide  gate,  one  form  of 
which  is  shown  in  Fig.  46,  is  a  special  form  of  check  valve  placed 
on  the  end  of  the  sewer. 

Sewer  outlets  are  sometimes  constructed  on  long  trestles  in 
order  to  rea'ch  deep  or  running  water.  Such  a  trestle  is  shown 
in  Fig.  47.  In  Boston  the  outlet  sewers  are  submerged  under  the 
harbor  and  discharge  through  outlets  well  out  in  the  tidal  currents. 
The  sewage  is  discharged  under  pressure  and  the  pumps  are 


OUTLETS 


123 


operated  at  some  of  the  stations  only  at  such  times  as  the  tidal 
currents  will  carry  the  sewage  away  from  the  harbor.  It  is  not 
always  necessary  in  a  combined  sewerage  system  to  carry  the 


Half  Cross  Section 


9     9      9  9 

Half  Elevation. 


6"fac/'ftg  Portl.  Concrete,  /'§••/£  Granite  Powder-^ 


Concrete  . 


^**Vl  *    ^  8  "Facing  PorH.  Concrete 
"***>          I  •'£  •'/£  Granite  Po  wder 


Longitudinal  Section. 
FIG.  47. — Sewer  Outlet  on  a  Trestle. 

Eng.  News,  Vol.  49,  p.  9. 

storm  flow  to  a  distant  submerged  outlet.  A  double  outlet  can 
be  constructed  as  shown  in  Fig.  48  in  which  the  dry-weather  flow 
is  carried  to  the  channel  in  a  submerged  sewer  and  the  storm 


124 


APPURTENANCES 


J 

/        n 

Extreme  H.W.+3.76 

El  +3  0 
1  —  I/'              r/_/rJo 

flow  is  discharged  on  the  bank.1  Cast-iron  pipe  should  be  used 
for  submerged  outlets  as  the  sewer  is  subject  to  disturbance  by 
the  currents,  anchors,  ice,  and  other  causes. 

66.  Foundations. — Sewers  constructed  in  firm  dry  soil  require 

no  special  foundation  to  dis- 
tribute the  weight  over  the  sup- 
porting medium.  In  soft  ma- 
terials the  lower  half  of  the 
sewer  ring  may  be  spread 
as  shown  in  Fig.  22,  and  in 
rock  the  pressures  on  sewer 
pipes  are  evenly  distributed 
by  a  cushion  of  sand.  In 

FIG.    48.-Dry    Weather    and    Storm   Wet      grOUnd     Such    as     ^u[ck' 

Sewer  Outlet  at  Minneapolis,  Min-   sand,  mud,    swamp    land,    etc., 

nesota.  a    foundation     must    be    con- 

Eng.  Record,  Vol.  63,  p.  383.  structed  if  the  water  cannot  be 

drained  off. 

The  permissible  intensities  of  pressure  on  foundations  in 
various  classes  of  material  allowed  by  the  building  codes  in  differ- 
ent cities  are  given  in  Table  25.  These  figures  are  based  on  the 
assumption  that  the  material  is  restrained  laterally,  which  is 
generally  the  condition  in  sewer  construction.  In  the  softer 
materials  it  becomes  necessary  to  spread  the  foundations  not 
only  to  reduce  the  intensity  of  pressure,  but  also  to  care  for  the 
thrust  of  the  sewer  arch.  The  arch  thrust  may  be  found  by  one 
of  the  methods  of  arch  analysis,  and  the  haunches  spread  to  care 
for  this,  or  the  sewer  invert  may  be  transversally  reinforced  to 
assist  in  caring  for  this  action.  Some  sewer  sections  in  hard  and 
soft  material  are  shown  in  Fig.  22  and  23. 

On  soft  foundations  such  as  swamps  or  for  outfalls  on  the  muck 
bottom  of  rivers  the  sewer  may  be  carried  on  a  platform.  For 
small  sewers  2-inch  planks,  2  to  4  feet  longer  than  the  diameter 
of  the  pipe  are  laid  across  the  trench,  and  the  sewer  rests  directly 
upon  them.  For  large  sewers  imposing  a  heavy  concentrated 
load,  a  pile  foundation  should  be  constructed.  The  foundation 
may  consist  of  piles  alone,  pile  bents,  or  a  wooden  platform  sup- 
ported on  pile  bents.  The  load  which  can  be  carried  by  a  pile  is 

1  Multiple  Outlet  for  Calumet  Intercepting  Sewer,  by  S.  T.  Smetters,  Eng. 
News-Record,  Vol.  83, 1919,  p.  728. 


FOUNDATIONS 

/ 

TABLE  25 
ALLOWABLE  BEARING  VALUE  ON  SOILS  IN  VARIOUS  CITIES 

From  Proc.  Am.  Soc.  Civil  Engrs.,  Vol.  46,  1920,  p.  906 


125 


Quicksand  and  alluvial  soil 

5  to  1  ton  per  sq.  ft.  for  Providence,  R.  I.,  5  ton  per 
sq.  ft.  for  6  cities 

Soft  clay 

1  ton  per  sq.  ft.  for  27  cities,  «-  ton  per  sq.  ft.  for  New 
Orleans,  2  to  3  tons  for  Providence,  R.  I. 

Moderately  dry  clay  and  fine  sand, 
clean  and  dry 

2  tons  for  7  cities,  If  to  2  i  for  Chicago,  2J  tons  for 
Louisville,  2  to  4  tons  for  Providence,  3  tons  for 
Grand  Rapids  and  Los  Angeles 

Clay  and  sand  in  alternate  layers 

2  tons  for  19  cities,  1J  to  2\  for  Chicago,  3  to  5  tons 
for  Providence 

Firm  and  dry  loam  or  clay,  or  hard 
dry  clay  or  fine,  sand 

3  tons  for  24  cities,  2$  tons  for  2  cities,  2  to  3  tons  for 
Atlanta,  3j  tons  for  Philadelphia,  4  tons  for  6  cities 

Compact  coarse  sand,  stiff  gravel  or 
natural  earth 

4  tons  for  25  cities,  3i  tons  for  Buffalo,  3  to  4  tons  for 
Atlanta,  4  to  5  tons  for  Cincinnati,  5  tons  for 
Denver,  4  to  6  tons  for  3  cities,  6  tons  for  Rochester, 

N.  Y. 

Coarse   gravel,   stratified   stone   and 
clay,  or  rock  inferior  to  best  brick 
masonry 

6  tons  for  3  cities,  5  tons  for  2  cities,  8  tons  for  1  city 

Gravel  and  sand  well  cemented 

8  tons  for  5  cities,  6  tons  for  2  cities,  8  to  10  tons  for 
1  city 

Good  hard  pan  or  hard  shale 

10  tons  for  4  cities,  6  tons  for  2  cities,  8  tons  for  1  city 

Good  hard  pan  or  hard  shale  unex- 
posed  to  air,  frost  or  water 

8  tons  for  1  city,  10  to  15  tons  for  1  city,  12  to  18 
tons  for  1  city 

Very  hard  native  bed  rock 

20  tons  for  5  cities,  15  tons  for  1  city,  10  tons  for 
1  city,  25  to  50  tons  for  1  city 

Rock  under  caisson 

24  tons  for  Baltimore,  25  tons  for  Cleveland 

determined  with  accuracy  only  by  driving  a  test  pile  and  placing 
a  load  on  it.  Where  piles  do  not  penetrate  to  a  firm  stratum  the 
load  they  will  support  can  be  determined  by  any  one  of  the  various 
formulas,  either  theoretical  or  empirical,  which  have  been  devised. 
Probably  the  best  known  of  these  formulas  are  the  so-called 
Engineering  News  formulas  one  of  which  is : 

P  =  -Q  TT  ^or  a  P^e  driven  by  a  drop  hammer, 


126  APPURTENANCES 

in  which    P  =  the  safe   load  on  the  pile  in  pounds.     The   factor 

of  safety  is  6; 

TF  =  the  weight  of  the  hammer  in  pounds; 
A  =  the  fall  of  the  hammer  in  feet; 

$=the  penetration  of  the  pile  in  inches  at  the  last 
driving  blow.  The  blow  is  assumed  to  be  driven 
on  sound  wood  without  rebound  of  the  hammer. 

Reference  should  be  made  to  engineering  handbooks  for  other 
forms  of  pile  formulas.  The  accuracy  of  all  of  these  formulas  is 
not  of  a  high  degree. 

The  piles  are  driven  at  about  2  to  4  feet  centers,  to  a  depth  of 
from  8  to  20  feet,  unless  hard  bottom  is  struck  at  a  lesser  depth. 
The  butt  diameter  of  the  piles  used  for  the  smallest  sewers  is 
about  6  to  8  inches.  If  bents  are  used,  2  or  3  piles  are  driven  in  a 
row  across  the  line  of  the  sewer  and  are  capped  with  a  timber. 
For  brick,  block,  pipe,  and  some  concrete  sewers,  a  wooden  plat- 
form must  be  constructed  between  the  pile  bents  for  the  support 
of  the  sewer. 

67.  Underdrains. — The  construction  of  special  foundations 
can  sometimes  be  avoided  by  laying  drains  under  the  sewers, 
thus  removing  the  water  held  in  the  soil.  The  laying  of  the  under- 
drains  facilitates  the  construction  of  the  sewer  and  reduces  the 
amount  of  ground  water  entering  the  sewer.  The  underdrains 
usually  consist  of  6-  or  8-inch  vitrified  tile  laid  with  open  joints 
from  1  to  2  feet  below  the  bottom  of  the  sewer  as  shown  in  Fig.  1. 
If  the  sewers  are  large,  parallel  lines  of  drains  may  be  laid  beneath 
them.  An  observation  hole  should  be  constructed  from  the  bottom 
of  the  manhole  to  each  underdrain.  This  hole  usually  consists 
of  a  6-  or  8-inch  pipe,  embedded  in  concrete,  connected  to  the 
drain  and  open  at  the  top.  It  is  too  small  to  permit  effective 
cleaning  of  the  underdrains,  which  are  usually  neglected  after 
construction,  and  which  as  a  result  clog  and  cease  to  function. 
Since  the  principle  period  of  usefulness  of  the  drains  is  during 
construction,  their  stoppage  after  the  work  is  completed  is  not 
serious.  The  hollow  tile  used  in  vitrified  block  sewers  serve  as 
underdrains  after  construction,  but  are  of  little  or  no  assistance 
to  the  draining  of  the  trench  during  construction, 


CHAPTER  VII 
PUMPS  AND  PUMPING  STATIONS 

68.  Need. — In  the  design  of  a  sewerage  system  it  is  occasion- 
ally necessary  to  concentrate  the  sewage  of  a  low-lying  district  at 
some  convenient  point  from  which  it  must  be  lifted  by  pumps. 
In    the    construction   of   sewers   in    flat    topography    the    grade 
required  to  cause  proper  velocity  of  sewage  flow  necessitates  deep . 
excavation.     It  is  sometimes  less  expensive  to  raise  the  sewage 
by  pumping  than  to  continue  the  construction  of  the  sewers  with 
deep  excavation. 

In  the  operation  of  a  sewage-treatment  plant  a  certain  amount 
of  head  is  necessary.  If  the  sewage  is  delivered  to  the  plant  at  a 
depth  too  great  to  make  possible  the  utilization  of  gravity  for  the 
required  head,  pumps  must  be  installed  to  lift  the  sewage.  In 
the  construction  of  large  office  buildings,  business  blocks,  etc., 
the  sub-basements  are  frequently  constructed  below  the  sewer 
level.  The  sewage  and  other  drainage  from  the  low  portion  of 
the  building  must  therefore  be  removed  by  pumping.  Because 
pumps  are  often  an  essenti.il  part  of  a  sewerage  system,  their 
details  should  be  understood  by  the  engineer  who  must  write  the 
specifications  under  which  they  are  purchased  and  installed. 

69.  Reliability. — If  the  only  outlet  from  a  sewerage  system  is 
through  a  pumping  station,  the  inability  of  the  pumps  to  handle 
all  of  the  sewage  delivered  to  them  may  so  back  up  the  sewage  as 
to  flood  streets  and  basements,  endangering  lives  and  health  and 
destroying   property.     Such   an   occurrence   should   be   guarded 
against  by  providing  sufficient  pumping  capacity  and  machinery 
of  the  greatest  reliability. 

70.  Equipment. — The  equipment  of  a  sewage  pumping  station, 
in  addition  to  pumping  machinery,  may  include  a  grit  chamber,  a 
screen,  and  a  receiving  well.     The  grit  chamber  and  screen  are 
necessaiy  to  protect  the  pumps  from  wear  and  clogging.     Grit 
chambers  are  not  necessary  in  sewage  devoid  of  gritty  matter, 

127 


128 


PUMPS  AND  PUMPING  STATIONS 


such  as  the  average  domestic  sewage,  unless  reciprocating  pumps 
are  used.  Sufficient  gritty  matter  is  found  in  average  domestic 
sewage  to  have  an  undesirable  effect  on  reciprocating  pumps. 
Receiving  wells  are  used  in  small  pumping  stations  where  the 
capacity  of  the  pumps  is  greater  than  the  average  rate  of  sewage 
flow.  The  pumps  are  then  operated  intermittently,  the  pumps 
standing  idle  during  the  time  that  the  receiving  well  is  filling. 

Except  for  a  few  types  of  pumps  of  which  the  valve  openings 
are  unsuitable,  any  machine  capable  of  pumping  water  is  capable 
of  pumping  sewage  which  has  been  properly  screened.  The 
principles  of  sewage  pumps  are  then  similar  to  principles  of  water 
pumps.  The  conditions  under  which  these  principles  are  applied 
differ  but  slightly  in  the  character  of  the  liquid,  and  a  smaller 
range  of  discharge  pressures.  Pumps  with  large  passages,  dis- 
charging under  low  heads  are  more  commonly  found  among 
sewage  pumps. 

71.  The  Building. — The  pumping  station  should,  if  possible, 
be  of  pleasing  design  and  should  be  surrounded  by  attractive 
grounds.  The  Calumet  Sewage  Pumping  Station  in  Chicago  is 
shown  in  Fig.  49.  Its  architecture  is  pleasing  particularly  in 


FIG.  49. — Calumet  Sewage  Pumping  Station,  Chicago,  Illinois. 

contrast  with  its  location  and  immediate  surroundings.  Such 
structures  tend  to  remove  the  popular  prejudice  from  sewerage 
and  to  arouse  interest  in  sewerage  questions.  Service  to  the 


CAPACITY  OF  PUMPS  129 

public  is  of  value.  It  can  be  rendered  more  easily  by  arousing 
public  interest  and  co-operation  by  the  erection  of  attractive 
structures,  than  by  feeding  popular  prejudice  by  the  construction 
of  miserable  eyesores. 

72.  Capacity  of  Pumps. — The  capacity  of  the  pumping  equip- 
ment should  be  sufficient  to  care  for  the  maximum  quantity  of 
sewage  delivered  to  it,  with  the  largest  pumping  unit  shut  down, 
and  the  provision  of  such  additional  capacity  as,  in  the  opinion 
of  the  designer,  will  provide  the  necessary  factor  of  safety. 

Pumps  can  usually  be  operated  under  more  or  less  overload. 
Power  pumps  and  centrifugal  pumps  driven  by  constant  speed 
electric  motors  have  no  overload  capacity.  A  power  pump  or  a 
centrifugal  pump  may  be  overloaded  up  to  the  maximum  horse- 
power of  any  variable  speed  motor  or  steam  engine  driving  it, 
provided  the  pump  has  been  designed  to  permit  it.  Direct-acting 
steam  pumps  which  are  designed  for  proper  piston  speed  and 
valve  action  at  normal  loads,  can  carry  a  50  per  cent  overload  for 
short  periods,  although  the  strain  on  the  pump  is  great.  They 
will  carry  a  20  to  25  per  cent  overload  for  about  eight  hours  with  less 
vibration  and  strain.  The  use  of  pumps  capable  of  working  at 
an  appreciable  overload  is  somewhat  of  an  additional  factor  "of 
safety,  but  the  overload  factor  should  not  be  taken  into  considera- 
tion in  determining  the  capacity  of  the  pumping  equipment. 

The  load  on  a  pumping  station  consists  of  the  quantity  of 
sewage  to  be  pumped  and  the  height  it  must  be  lifted.  Variations 
in  the  quantity  are  discussed  in  Chapter  III.  The  head  against 
which  the  pumps  must  operate  fluctuates  with  the  level  in  the 
tributary  sewer  or  pump  well,  and  in  the  discharge  conduit. 
For  a  free  discharge  or  discharge  into  a  short  force  main  the  greater 
the  rate  of  sewage  flow  the  smaller  the  lift,  as  the  depth  of  flow 
in  the  tributary  sewer  increases  more  rapidly  than  that  in  the 
discharge  conduit.  If  the  discharge  is  into  a  large  body  of  water 
or  under  other  conditions  where  the  discharge  head  is  approxi- 
mately constant,  the  fluctuations  in  total  head  should  not  exceed 
the  diameter  of  the  tributary  sewer.  Such  fluctuations  are  of 
minor  importance  in  the  operation  of  direct-acting  steam  pumps, 
but  may  be  of  great  importance  in  the  operation  of  centrifugal 
pumps,  as  is  brought  out  in  Art.  78. 

73.  Capacity  of  Receiving  Well. — The  use  of  receiving  wells 
is  restricted  to  small  installations  which  require,  in  addition  to 


130  PUMPS  AND  PUMPING  STATIONS 

the  standby  unit,  only  one  pump,  the  capacity  of  which  is  equal 
to  the  maximum  rate  of  sewage  flow.  When  the  receiving  well 
has  been  pumped  dry  the  pump  stops,  allowing  the  well  to  fill 
again.  Although  the  use  of  a  large  receiving  well,  or  an  equaliz- 
ing reservoir,  for  a  large  pumping  station  would  permit  the  opera- 
tion of  the  pumps  under  more  economical  conditions,  the  storage 
of  sewage  for  the  length  of  time  required  would  not  be  feasible. 
The  sewage  would  probably  become  septic,  creating  odors  and 
corroding  the  pumps.  The  extra  cost  of  the  reservoir  might  not 
compensate  for  the  saving  in  the  capacity  and  operation  of  the 
pumps. 

The  capacity  of  the  receiving  well  should  be  so  designed  that 
the  pump  when  operating  will  be  working  at  its  maximum  capacity, 
and  the  period  of  rest  during  conditions  of  average  rate  of  flow 
should  be  in  the  neighborhood  of  15  to  20  minutes.  For  example, 
assume  an  average  rate  of  flow  of  2  cubic  feet  per  second,  with  a 
maximum  rate  of  double  this  amount.  The  pump  should  have  a 
capacity  of  4  cubic  feet  per  second,  and  if  the  receiving  well  is  to 
be  filled  in  15  minutes  by  the  average  rate  of  sewage  flow  its  capac- 
ity should  be  15X5X60X7.5  or  14,000  gallons.  Under  these 
circumstances  the  pump  will  operate  15  minutes  and  rest  15 
minutes,  during  average  conditions  of  flow. 

74.  Types  of  Pumping  Machinery. — The  two  principal  types 
of  pumping  machines  for  lifting  sewage  are  centrifugal  pumps  and 
reciprocating  pumps.  A  centrifugal  pump  is,  in  general,  any 
pump  which  raises  a  liquid  by  the  centrifugal  force  created  by  a 
wheel,  called  the  impeller,  revolving  in  a  tight  casing,  as  shown  in 
Fig.  50.  A  reciprocating  pump  is  one  in  which  there  is  a  periodic 
reversal  of  motion  of  the  parts  of  the  pump. 

Centrifugal  pumps  are  sometimes  classified  as  volute  pumps 
and  turbine  pumps.  A  volute  pump  is  a  centrifugal  pump  with  a 
spiral  casing  into  which  the  water  is  discharged  from  the  impeller 
with  the  same  velocity  at  all  points  around  the  circumference,  as 
shown  in  Fig.  51.  A  turbine  pump  is  a  centrifugal  pump  in  which 
the  water  is  discharged  from  the  impeller  through  guide  passages 
into  a  collecting  chamber,  in  such  a  manner  as  to  prevent  loss  of 
energy  in  changing  from  kinetic  head  to  pressure  head.  A  tur- 
bine pump  is  shown  in  section  in  Fig.  51.  Centrifugal  pumps  are 
sometimes  classified  as  single  stage  and  multi-stage.  A  centrif- 
ugal pump  from  which  the  water  is  discharged  at  the  pressure 


TYPES   OF   PUMPING   MACHINERY 


131 


created  by  a  single  impeller  is  called  a  single-stage  pump.  If  the 
water  in  the  pump  is  discharged  from  one  impeller  into  the  suction 
of  another  impeller  the  pump  is  known  as  a  multi-stage  pump. 


FIG.  50. — Section  through  de  Laval  Single-Stage,  Double-Suction  Centrifugal 

Pump. 


375  Lubricating  ring.  554 

380  Oil  hole  cap.  555 

383  Oil  drain  tube.  555-1 

404  Shaft  sleeve  lock  nut.  556 

440  Driving  coupling.  560 

441  Driven  coupling.  563 
443  Coupling  check  nut.  567  R 

450  Coupling  bolt. 

451  Coupling  bolt  nut.  567L 

452  Coupling  rubber. 

453  Coupling  rubber  steel  tube.  583 
500  Pump  case.  567  \ 

550  Bearing  bracket  cap.  583  / 

551  Bearing.  600 

552  Shaft.  692 

553  Shaft  sleeve,  right  hand  thread  815 
PW  Impeller.  815-1 


Shaft  sleeve,  left  hand  thread. 

Shaft  stop  collar,  inner. 

Shaft  stop  collar,  outer. 

Guide  ring. 

Packing  gland. 

Bearing. 

Impeller   protecting   ring,    right   hand 

thread. 
Impeller    protecting    ring,  left    hand 

thread. 
Pump  case  protecting  ring. 

Labyrinth  packing. 

Pump  case  cover. 
Impeller  key. 
Bearing  bracket,  outer. 
Bearing  bracket,  inner. 


The  number  of  impellers  operating  at  different  pressures  deter- 
mines the  number  of  stages  of  the  pump.  A  three-stage  pump  is 
shown  in  Fig.  52. 


132 


PUMPS  AND  PUMPING  STATIONS 


Reciprocating  pumps  are  generally  driven  by  steam  and  are 
either  direct-acting,  or  of  the  crank-and-fly-wheel  type.  Power 
pumps  are  reciprocating  machines  which  may  be  driven  by  any 
form  of  motor,  to  which  they  are  connected  by  belt,  chain  or  shaft. 
A  Deming  triplex  power  pump  is  shown  in  Fig.  53.  Power 


--Impeller 

-^Diffusion 
Vanes 


Pump.  Turbine  Pump,     Circular  Case. 

FIG.  51. — Types  of  Centrifugal  Pumps. 


Volute 


pumps  can  be  used  only  where  the  character  of  the  sewage  will 
not  clog  the  valves  nor  corrode  the  pump.  A  direct-acting  steam 
pump  is  one  in  which  the  steam  and  water  cylinders  are  in  the 
same  straight  line  and  the  steam  is  used  at  full  boiler  pressure 
throughout  the  full  length  of  the  stroke.  The  crank-and-fly- 


FIG.  52. — Section  of  a  Multi-Stage  Centrifugal  Pump. 

Courtesy  DeLaval  Steam  Turbine  Co. 

wheel  type  of  pumping  engine  permits  the  use  of  steam  expan- 
sively during  a  part  of  the  stroke,  the  energy  stored  in  the  fly- 
wheel carrying  the  machine  through  the  remainder  of  the  stroke. 
Reciprocating  pumps  are  sometimes  classified  as  plunger  pumps 
and  piston  pumps.  In  the  action  of  a  plunger  pump  the  water  is 
expelled  from  the  water  cylinder,  by  the  action  of  a  plunger 


TYPES  OF  PUMPING  MACHINERY 


133 


which  only  partly  fills  the  water  cylinder,  as  shown  in  Figs.  54 
and  55.  In  a  piston  pump  the  water  is  expelled  from  the  water 
cylinder  by  the  action  of  a  piston  which  completely  fills  the  water 
cylinder,  as  shown  in  Fig. 
63,  which  illustrates  a  direct- 
acting  piston  pump. 

Plungers  are  better  than 
pistons  for  pumping  sewage 
as  the  wear  between  the  pis- 
tons and  the  inside  face  of 
the  cylinder  soon  reduces  the 
efficiency  of  the  pump.  Out- 
side packed  plungers  are 
better  than  the  inside  packed 
type  because  the  packing  can  FlG  53.— Triplex  Power  Pump. 

be  taken  Up  without  Stopping  Courtesy,  The  Deming  Co. 

the  pump  and    the   leakage 

from  the  pump  is  visible  at  all    times.     Outside   packed  pumps 

are  more  expensive  in  first  cost,  but  are  easier  to  maintain  and 

have  a  longer  life  than  piston  pumps. 

In  selecting  a  pump  to  perform  certain  work  the  size  of  the 

water  cylinder  and  the  speed  of  the  travel  of  the  piston  should  be 

investigated  to  insure  proper 
capacity.  The  average  linear 
travel  of  the  piston  for  slow 
speed  pumps  is  estimated  at 
about  100  feet  per  minute, 
dependent  on  the  length  of 
stroke  and  the  valve  area. 
For  short  strokes  and  small 
valve  areas  the  speed  may 
be  as  low  as  40  feet  per  min- 
ute, and  for  long  stroke  fire 
pumps  with  large  valves 


FIG.  54. — Water  End  of  Inside  Center- 
Packed  Plunger  Pump. 


the  piston  can   be  operated 
at  a  speed  of  200    feet  per 
minute.1     Vertical,  triple-expansion,  crank-and-fly-wheel,  outside- 
packed  plunger  pumps  with  flap  valves  can  be  operated  at  speeds 
of  200  feet  per  minute  when  lifting  sewage,  and  when  equipped 
1  "  Direct  Acting  Steam  Pumps,"  by  F.  R.  Nickel,  1915. 


134 


PUMPS  AND  PUMPING  STATIONS 


with  mechanically  operated  valves  and  lifting  water  they  can  be 
run  at  speeds  of  400  to  500  feet  per  minute.  The  speed  of  travel 
multiplied  by  the  volume  of  piston  or  plunger  displacement,  with 
proper  allowance  for  slippage,  will  give  the  capacity  of  the  pump. 
The  slippage  allowance  may  be  from  3  to  8  per  cent  for  the  best 
pumps,  and  for  pumps  in  poor  conditions  it  may  be  a  high  as  30  to 
40  per  cent. 


Channel  Way 
—  to  Air 
Pump 


Inlet  from 
Main  Suction  Pipe 


FIG.  55 — Water  End  of  Outside  Center-Packed  Plunger  Pump. 

Courtesy  Allis-Chalmers  Co. 


There  are  two  types  of  ejector  pumps  used  for  lifting  sewage. 
One  of  these  depends  on  the  vacuum  created  by  the  velocity  of  a 
stream  of  water  or  steam  passing  through  a  small  nozzle.  The 
operation  of  this  pump  is  described  in  Art.  139  and  it  is  illustrated 
in  Fig.  97.  The  other  type  of  ejector  pump  is  known  as  the  com- 
pressed-air ejector.  It  is  operated  by  means  of  compressed  air 
which  is  turned  into  a  receptacle  containing  sewage.  The  details 
of  this  type  are  explained  in  Art.  83  and  are  illustrated  in  Fig.  68. 


SIZES  AND  DESCRIPTION  OF  PUMPS  135 

75.  Sizes  and  Description  of  Pumps. — The  size  of  a  centrif- 
ugal pump  is  expressed  as  the  diameter  of  the  discharge  pipe  in 
inches.     It  has  nothing  to  do  with  the  head  for  which  the  pump 
is  suited.     On  the  assumption  of  a  velocity  of  flow  of  10  feet  per 
second  through  the  discharge  pipe  the  capacity  of  the  pump  can  be 
approximated. 

The  size  of  a  reciprocating  pump  involves  the  expression  of  the 
diameters  of  the  steam  cylinders,  the  water  cylinder,  and  the  length 
of  the  stroke  in  inches,  in  the  order  named,  beginning  with  the 
steam  cylinder  with  the  highest  pressure.  A  complete  descrip- 
tion of  a  steam  pumping  engine  might  be;  'compound,  duplex, 
horizontal,  condensing,  crank-and-fly-wheel,  outside-center- 
packed,  12"X24"X18"X24"  pump.  The  word  compound 
means  that  there  are  a  high-pressure  and  a  low-pressure  steam 
cylinder;  the  word  duplex  means  that  there  are  two  of  each  of 
these  cylinders;  the  word  horizontal  means  that  the  axes  of  these 
cylinders  are  in  a  horizontal  plane;  the  word  condensing  means 
that  the  steam  is  discharged  from  the  low-pressure  cylinders  into  a 
condenser;  the  name  crank-and-fly-wheel  is  self-explanatory; 
the  name  outside-center-packed  means  that  the  water  cylinder  is 
divided  into  two  portions  between  which  the  plunger  is  exposed 
to  the  atmosphere,  and  that  the  packing  rings  are  on  the  outside 
of  the  two  portions  of  the  cylinder  as  shown  in  Fig.  55 ;  the  figures 
shown  mean  that  the  high-pressure  steam  cylinder  is  12  inches  in 
diameter,  the  low-pressure  24  inches  in  diameter,  the  water  cylin- 
der is  18  inches  in  diameter,  and  the  stroke  of  the  pump  is  24 
inches. 

76.  Definitions  of  Duty  and  Efficiency. — The  duty  of  a  pump 
is  the  number  of  foot  pounds  of  work  done  by  the  pump  per 
million  B.T.U.,  per  thousand  pounds  of  steam,  or  per  hundred 
pounds  of  coal,  consumed  in  performing  the  work.     These  units 
are  only  approximately  equal  as  100  pounds  of  coal  or  1,000  pounds 
of  steam  do  not  always  contain  the  same  number  of  B.T.U.  and 
may  only  approximately  equal  1,000,000  B.T.U. 

Since  1,000,000  B.T.U.  are  equal  to  778,000,000  foot-pounds 
of  work,  a  pump  with  a  duty  of  778,000,000  will  have  an  effi- 
ciency of  100  per  cent.  The  efficiency  of  a  pump  is  therefore  its 
duty  based  on  B.T.U.  divided  by  778,000,000.  The  efficiencies 
or  duties  of  various  types  of  pumps  are  given  in  Table  26.1 
1  From  Heat  Engines,  by  Allen  and  Bursley. 


136  PUMPS  AND  PUMPING  STATIONS 

TABLE  26 
APPROXIMATE  DUTIES  OF  STEAM  PUMPS 

Small  duplex,  non-condensing 10,000,000 

Large  duplex,  non -condensing 25,000,000 

Small  simple,  flywheel,  condensing 50,000,000 

Large  simple,  flywheel,  condensing 65,000,000 

Small  compound,  flywheel,  condensing 65,000,000 

Large  compound,  flywheel,  condensing 120,000,000 

Small  triple,  flywheel,  condensing 150,000,000 

Large  triple,  flywheel,  condensing 165,000,000 

77.  Details  of  Centrifugal  Pumps. — A  section  of  a  centrifugal 
pump  with  the  names  of  the  parts  marked  thereon  is  shown  in 
Fig.  50.  Among  the  important  parts  which  require  the  attention 
of  the  purchaser  are:  the  impeller  (PW),  the  impeller  packing 
rings  (567  R  &  L),  the  bearings  (551,  563),  the  thrust  bearings 
(555-1),  the  shaft  (552),  and  the  shaft  coupling  (440). 

The  impeller  should  be  of  bronze,  gun  metal,  or  other  alloy, 
because  there  is  no  rusting  or  roughening  of  the  surface,  and  the 
efficiency  does  not  fall  with  age.  Normal  fresh  sewage  is  not 
corrosive,  but  septic  sewage  and  sludge  are  usually  so  corrosive 
that  iron  parts  cannot  be  used  with  success  in  contact  with  them. 
The  impeller  should  be  machined  and  polished  to  reduce  the  fric- 
tion with  the  liquid.  Impellers  are  made  either  closed  or  open, 
i.e.,  either  with  or  without  plates  on  the  sides  connecting  the 
blades  to  avoid  the  friction  of  the  liquid  against  the  side  of  'the 
casing.  The  closed  type  of  impeller  is  shown  in  Fig.  50.  Closed 
impellers  are  slightly  more  expensive,  but  generally  give  better 
service  and  higher  efficiencies  than  the  open  type.  Single  impeller 
pumps  should  have  an  inlet  on  each  side  of  the  impeller  to  aid  in 
balancing  the  machine,  unless  the  plane  of  the  impeller  is  to  be 
horizontal  when  operating.  Multi-impeller  pumps  usually  have 
single  inlet  openings  for  each  impeller.  Vibration  in  the  pump  is 
sometimes  caused  by  an  unbalanced  impeller.  The  moving  parts 
may  be  balanced  at  one  speed  and  unbalanced  at  another.  To 
determine  if  the  moving  parts  are  balanced  the  pump  should  be 
run  free  at  different  speeds  and  the  amount  of  vibration  observed. 
If  the  impeller  is  removed  from  the  pump  its  balance  when  at 
rest  can  be  studied  by  resting  it  on  horizontal  knife  edges.  If 
there  is  a  tendency  to  rotate  in  any  direction  from  any  position 
the  impeller  is  not  perfectly  balanced. 


DETAILS  OF  CENTRIFUGAL  PUMPS  137 

Packing  rings  are  used  to  prevent  the  escape  of  water  from 
the  discharge  chamber  back  into  the  suction  chamber.  These 
rings  should  be  made  of  the  same  material  as  the  impeller. 
Labyrinth  type  rings,  as  shown  in  Fig.  50,  are  sometimes  used  as 
the  long  tortuous  passages  are  efficient  in  preventing  leakage. 

The  bearings  must  be  carefully  made  because  of  the  high  speed 
of  the  pump.  They  are  usually  made  of  cast  iron  with  babbitt 
lining.  They  should  be  placed  on  the  ends  of  the  shaft  on  the 
outside  of  the  pump  casing,  as  shown  in  Fig.  50,  and  should  be 
split  horizontally  so  as  to  be  easily  renewed.  Exterior  bearings 
are  oil  lubricated  by  means  of  ring  or  chain  oilers  with  deep  oil 
wells.  Where  interior  bearings  are  necessary,  because  of  the  length 
of  the  shaft,  they  should  be  made  of  hard  brass  and  should  be 
water  lubricated. 

Thrust  bearings  or  thrust  balancing  devices  are  used  to  take 
up  the  end  thrust  which  occurs  in  even  the  best  designed  pumps. 
To  overcome  this  pumps  are  designed  with  double  suction, 
opposed  impellers,  or  two  pumps  with  their  impellers  opposed 
may  be  placed  on  the  same  shaft.  Due  to  inequalities  in  wear, 
workmanship  or  other  conditions,  end  thrust  will  occur  and  must 
be  cared  for.  Various  types  of  thrust  bearings  are  in  successful 
use,  such  as:  the  piston,  ball,  roller  or  marine  types.  The  marine 
type  thrust  bearing  is  shown  in  Fig.  56.  The  piston  type  of 


FIG.  56. — Marine  Type  Thrust  Bearing. 
Courtesy,  DeLaval  Steam  Turbine  Co. 

hydraulic  balancing  device  is  shown  in  Fig.  57.  In  the  figure  A 
represents  the  impeller,  and  B  a  piston  fixed  to  the  shaft  and 
revolving  with  it.  There  is  a  passage  for  water  through  the  open- 
ings (1),  (2),  and  (3)  leading  from  the  impeller  chamber  to  the 
atmosphere  or  to  the  suction  of  the  pump.  If  the  impeller  tends 
to  move  to  the  right  opening  (1)  is  closed  resulting  in  pressure  on 


138  PUMPS  AND  PUMPING  STATIONS 

the  right  of  the  impeller  forcing  it  to  the  left.  If  the  impeller 
moves  to  the  left  (1)  is  opened  thus  transmitting  pressure  to  'the 
piston  B  forcing  the  impeller  to  the  right.  The  flange  C  is  not 
essential,  but  is  advantageous  in  pumps  handling  gritty  matter. 
As  the  channel  (2)  wears  larger  the  pressure  against  the  piston 
decreases  allowing  it  to  move  to  the  left.  This  partially  closes 
(3)  building  up  the  pressure  again. 

Flexible  shaft  couplings  should  be  used  if  the  shaft  of  the 
driving  motor  and  the  pump  are  in  the  same  line,  as  direct  align- 
ment is   difficult   to   obtain   or  to 
maintain.  Where  connected  to  steam 
turbines,  reduction  gearing  and  rigid 
couplings  are  usually  used  on  sewage 
G  pumps  to  obtain  slow  speed  and  per- 

'i  2''H~  mit  large  passages.     Flexible  coup- 

FIG.  57.— Piston  Type  of  Thrust    lingsare  of  various  types,  oneof  which 
Balancing  Device.  is  shown  in  Fig.  50.     A  rigid  coup- 

ling would  be  formed  by   bolting 

the  flanges  firmly  together.  Shaft  couplings  are  usually  not 
necessary  where  the  pump  is  driven  by  belt  connection  to  the 
engine  or  motor,  or  where  the  pump  and  pulley  rest  on  only  two 
bearings. 

The  stuffing  box  shown  in  Fig.  50  is  packed  loosely  with  two 
layers  of  hemp  between  which  is  a  lantern  gland,  in  order  to  permit 
a  small  amount  of  leakage.  A  drip  box  is  placed  below  this  gland 
to  catch  the  leakage  and  return  it  to  the  pump.  The  leakage  is 
permitted  as  it  aids  in  lubrication  and  the  tightening  of  the  gland 
will  cause  binding  of  the  shaft.  The  gland  on  the  suction  side  of 
the  pump  should  be  connected  by  a  small  pipe  to  the  discharge 
chamber  in  order  to  keep  a  constant  supply  of  water  for  lubrica- 
tion and  to  prevent  the  entrance  of  air  to  the  suction  end  of  the 
pump. 

78.  Centrifugal  Pump  Characteristics. — The  capacity  of  a 
centrifugal  pump  is  fixed  by  the  size  and  type  of  its  impeller  and 
by  the  speed  of  revolution.  Roughly,  the  capacity  of  a  pump, 
for  maximum  efficiency,  varies  directly  as  the  speed  of  revolution, 
the  discharge  pressure  varies  as  the  square  of  the  speed,  and  the 
power  varies  as  the  cube  of  the  speed.  These'  relations  are  found 
not  to  hold  exactly  in  tests  because  of  internal  hydraulic  friction 
in  the  pump. 


CENTRIFUGAL  PUMP  CHARACTERISTICS 


139 


The  characteristic  curves  for  a  centrifugal  pump,  or  the  so- 
called  pump  characteristics,  are  represented  graphically  by  the 
relation  between  quantity  and  efficiency,  quantity  and  power 
necessary  to  drive,  and  quantity  and  head,  all  at  the  same  speed. 
The  quantities  are  plotted  as  abscissas  in  every  case.  The  curve 
whose  ordinates  are  head  and  whose  abscissas  are  quantities  is 
known  as  "  the  characteristic."  The  curve  showing  the  relation 
between  quantities  and  speeds  is  sometimes  included  among  the 
characteristics.  Characteristics  of  pumps  with  different  styles 
of  impellers  are  shown  in  Fig.  58.  Fig.  59  shows  the  character- 
istics of  a  pump  run  at  different  speeds,  the  efficiencies  at  these 


Type  L 


10      20       30      40       50      60       70      80 
Capacity  in  Gallons  per  Minute 

FIG.  58. — Characteristics  of  Centrifugal  Pumps  with  Different  Styles  of 
Impellers  at  Constant  Speed. 

speeds  when  pumping  at  different  rates,  and  the  maximum  effi- 
ciency at  different  speeds.  It  is  to  be  noted  that  the  informa- 
tion given  in  this  figure  is  more  extensive  than  that  in  Fig.  58. 
The  operating  conditions  under  any  head,  rate  of  discharge,  and 
speed  are  given.  The  curves  of  constant  speed  are  parallel,  and 
their  distances  apart  vary  as  the  square  of  the  speed.  The 
line  of  maximum  efficiency  is  approximately  a  parabola. 

A  study  of  the  characteristics  of  any  particular  pump  should  be 
made  with  a  view  to  its  selection  for  the  load  and  conditions  under 
which  it  is  to  be  used.  Among  the  important  things  to  be  con- 
sidered in  the  selection  of  a  centrifugal  pump  for  the  expected 
conditions  of  load  are:  the  capacity  required,  the  maximum  and 
minimum  total  head  to  be  pumped  against,  the  maximum  varia- 
tions in  suction  and  discharge  heads,  and  the  nature  of  the  drive. 
For  example,  the  pump,  whose  characteristics  are  shown  in  Fig. 


140 


PUMPS  AND  PUMPING  STATIONS 


59,   should  be  operated  at  about  800  revolutions  per  minute. 
Under  total  heads  between  40  and  50  feet,  the  discharge  for  the 


100    200    500     400    500  .600    700     800     900     1000    1100     1200    1300 
Capacity  in  Gallons  per^Minute 

FIG.  59.  —  Efficiency  and  Characteristic  Curves  of  a  Centrifugal  Pump  at 

Different  Speeds. 

best    efficiency    will    vary   between    600   and   670   gallons   per 

minute. 

The  efficiencies  of  centrifugal  pumps  increase  with  their 
capacities  as  is  shown  approximately 
in  Fig.  60. 

79.  Setting  of  Centrifugal  Pumps. 
—  In  setting  a  centrifugal  pump,  care 
should  be  taken  to  provide  a  firm 
foundation  to  hold  the  shafts  of  the 
pump  and  the  electric  motor  or  the 
reduction  gearing  in  good  alignment, 


20 


Size  of  Pumps  in  Inches 


30     40     so    or  to  prevent  the   pump   from  being 


FIG.  60. — Efficiencies  of  Cen- 
trifugal Pumps. 


displaced  by  the  pull  of  a  belt.  It  is 
desirable  that  the  foundation  be  level. 
Centrifugal  pumps  should  be  set  sub- 
merged for  small  pumping  stations 

automatically  controlled.     Sludge  pumps  must  be  set  submerged 
as  otherwise  they  will  not  prime  successfully.     Provision  should  be 


SETTING  OF  CENTRIFUGAL  PUMPS 


141 


made  by  which  the  pump  can  be  lifted  from  the  sewage,  or  sludge, 
for  inspection  and  repair.  In  many  cases  the  pump  can  be  made 
self -priming  by  setting  it  in  a  dry.  water-tight  vault  below  the  low 
level  of  sewage  flow.  Where  possible  it  is  desirable  not  to  set  the 
pump  submerged  as  it  will  receive  better  care  when  easily  acces- 
sible. 

The  suction  pipe  should  be  free  from  vertical  bends  where  air 
might  collect  and  should  be  straight  for  at  least  18  to  24  inches 
from  the  pump  casing.  An  elbow  on  the  suction  pipe,  attached 
directly  to  the  casing  of  the  pump  gives  a  lower  efficiency  than  a 
suction  pipe  with  a  short  straight  run.  Centrifugal  pumps  will 
operate  with  as  high  a  suction  lift  as  reciprocating  pumps,  but  at 
the  start  they  must  be  primed  and  some  provision  must  be  made 
for  priming  them.  The  suction  pipe  should  be  equipped  with 
foot  valves  to  hold  the  priming,  or  some  method  may  be  provided 
for  exhausting  the  air  from  the  suction  pipe.  The  foot  valves 
should  be  so  installed  as  to  form  no  appreciable  obstruction  to  the 
flow  of  water.  They  should  have  an  area  of  opening  at  least  50 
per  cent  greater  than  the  cross-section  of  the  suction  pipe.  A 
strainer  on  the  suction  pipe  is  un- 
desirable as  it  becomes  clogged 
and  is  usually  in  an  inaccessible 
position  for  cleaning.  A  screen 
should  be  placed  at  the  en- 
trance to  the  suction  well  to 
prevent  the  entrance  of  objects 
that  are  likely  to  clog  the  pump. 
A  gate-valve  and  a  check-valve 
should  be  provided  on  the  dis- 
charge pipe,  the  former  to  assist 
in  controlling  the  rate  of  dis- 
charge and  the  latter  to  prevent 
back  flow  into  the  pump  when 
it  is  not  operating. 

Centrifugal    pumps  are  well  FlG   61. -Centrifugal  Pump  in  Man- 
adapted  to  service  in  either  large  hole  at  Duluth,  Minn, 
or  small  units.     An  installation      Eng.  Contracting,  vol.  43, 1915,  P.  310. 
in  a  manhole  at    Park  Point, 

Duluth,  is  shown  in  Fig.  61.  This  station  is  controlled  by  an 
automatic  electric  device  which  is  operated  by  a  float  in  the  suc- 


Ventilator 


24"M.H. 
Cover. 


Float- 


142 


PUMPS  AND  PUMPING  STATIONS 


tion  pit.  Such  automatic  control  is  an  added  advantage  of  the 
use  of  electrically  driven  centrifugal  pumps.  The  Calumet 
Pumping  Station  in  Chicago,  shown  in  Fig.  49,  has  a  capacity  of 
approximately  1,000  cubic  feet  per  second.  The  simplicity  of  the 
layout  of  this  station  is  shown  in  Fig.  62. 


FIG.  62. — Interior  Arrangement  of    the  Calumet  Sewage  Pumping  Station, 

Chicago. 

Eng.  News-Record,  Vol.  85,  1920,  p.  872. 


80.  Steam  Pumps  and  Pumping  Engines. — The  direct-acting 
steam  pump,  one  type  of  which  is  shown  in  Fig.  63,  is  adapted  to 
pumping  sewage  the  character  of  which  will  not  corrode  or  clog 
the  valves.  In  this  form  of  pump  it  is  necessary  to  utilize  the 
steam  at  full  pressure  throughout  the  entire  length  of  the  stroke, 
which  results  in  high  steam  consumption.  A  fly-wheel  permits 
the  use  of  steam  expansively  during  a  part  of  the  stroke,  thus 
increasing  the  economy  of  operation.  Other  devices  used  for  the 
same  purpose  are  known  as  compensators.  They  are  not  in 
general  use. 

Steam  engines  are  classified  in  many  different  ways,  for 
example ;  according  to  the  type  of  valve  gear,  as,  plain  slide  valve, 
Corliss,  Lentz,  etc.;  or  according  to  the  number  of  steam  expan- 


STEAM   PUMPS  AND  PUMPING  ENGINES 


143 


sions,  as,  simple,  compound,  triple  expansion,  etc.;  or  according 
to  the  efficiency  of  the  machine  as  low  duty  or  high  duty;   or  as 


152 


FIG.  63. — Section  of  Duplex -Piston  Steam  Pump. 

Courtesy,  The  John  H.  McGowan  Co. 


STEAM  END 
2  Steam  cylinder  and  housing  combined. 

8  Steam  piston  head. 

9  Steam  piston  follower. 

10  Steam  piston  inside  ring. 

11  Steam  piston  outside  ring  (2). 

12  Steam  cylinder  head. 
14  Steam  chest. 

16  Steam  chest  cover. 

17  Steam  slide  valve. 

18  Steam  valve  rod. 

20  Steam  valve  rod,  pin  and  nut. 

22  Steam  valve  rod,  collar  and  set  screw. 

23  Steam  valve  rod,  stuffing  box. 

24  Steam  valve  rod,  stuffing  box,  nut  and 

gland. 
38  Piston  rod. 

47  Piston  rod  stuffing  box. 

48  Piston  rod,  stuffing  box,  nut  and  gland. 

49  Valve  gear  stand. 

51  Long  valve  crank  and  shaft. 

52  Short  valve  crank  and  shaft. 


PUMP  END 
115  Pump  body. 
127  Brass  liner. 

129  Water  piston  head. 

130  Water  piston  follower. 
137  Cylinder  head. 

139  Valve  plate. 

140  Cap 

152  Suction  flange. 

161  Discharge  flange. 

162  Valve  seat,  suction  or  discharge. 

163  Valve,  suction  or  discharge. 

164  Suction  valve  spring. 

167  Discharge  valve  spring. 

168  Valve  plate,  suction  or  discharge. 

169  Valve  stem,  suction  or  discharge. 

STEAM  END 

55  Crank  pin. 

56  Valve  rod  link. 

61  Long  rocker  arm. 

62  Short  rocker  arm. 

63  Rocker  arm  wiper. 
69  Cross  head. 


condensing  or  non-condensing,  etc.  Throttling  engines  or  auto- 
matic engines  refer  to  the  method  of  control  of  the  steam  by  the 
governor.  In  throttling  engines  the  governor  controls  the  amount 


144 


PUMPS  AND  PUMPING  STATIONS 


of  opening  of  the  throttle  valve,  in  automatic  engines  the  governor 
controls  the  position  of  the  cut-off. 

The  simple  slide-valve,  low-duty,  non-condensing,  throttling 
engine,  is  the  lowest  in  first  cost  and  the  most  expensive  in  the 
consumption  of  fuel.  The  triple-expansion  Corliss,  or  the  non- 
releasing  Corliss,  high-duty  pumping  engine  is  the  most  expensive 
in  first  cost  but  consumes  less  steam  for  the  power  delivered  than 
any  other  form  of  reciprocating  engine.  For  pumps  of  very  small 
capacity  the  cost  of  fuel  is  not  so  important  an  item  as  the  first 
cost  of  the  machine.  For  this  reason  and  because  of  the  lower 


9x8  Non  Condensing,  Jhrottlinq 
'  'iqh  Speed,  Single  Valve 


125  K.  W.  Non  Condensing,  Aufomah'c, 
High  Speed   Single  Valve 


75 K.  W.  Condensing,  ffofary  4  Valve,  MediumSpeei 


125 K.W.  Compound  Condensing, 
Automatic,  HighSpeed,SmgleValve- 


400 K.W. Compound" 


Condensing,  Medium  Speed,  Popperr  Valve 


40  60  80 

Per  Cent  of  Rated  Load 


I.I  ll| 

Condensing,  Medium  Speed,  Rotary  4  Valve 


Compound  Condensing,  Medium  Speed,  Poppetr  Valve 


600  &00 

Horse    Power 

FIG.  64. — Diagram  Showing   Rates  of  Steam  Consumption  for  Different  Size 
Units  under  Different  Loads. 


cost  of  attendance  low-duty  pumps  are  more  frequently  found  in 
small  pumping  stations. 

The  steam  consumption  per  indicated  horse-power,  better 
known  as  the  water  rate  of  the  engine,  for  various  types  of  engines 
at  full  and  at  part  load  is  shown  in  Fig.  64.  The  steam  consump- 
tion of  other  types  at  full  load  is  shown  in  Table  27.  The  indi- 
cated horse-power  (I.H.P.)  of  a  steam  engine  is  the  product  of 
the  mean  effective  pressure  (M.E.P.),  the  area  of  the  steam 


STEAM  PUMPS  AND  PUMPING  ENGINES 


145 


TABLE  27 
WATER  RATES  OF  PRIME  MOVERS  AT  FULL  AND  PART  LOADS 


Power, 

I 

>er  Cei 

it  of  F 

ull  Loa 

i 

Boiler 

Type  of  Engine 

K.W. 

25 

50 

75 

100 

125 

Press, 
Lbs. 

Single  cylinder,  high  speed,  non-condensing 

25 
250 

33 

42 

27 
37.5 

26.3 
35 

27.0 
34.0 

27.5 
34.0 

100  to 
150 

Automatic,  flat  four  valve,  high  speed 

150 
250 

32 
33 

30 
31 

26.5 

28 

29.0 
30.0 

100  to 
125 

Tandem  compound  condensing,  high  speed 

125 

23 
25 

19 
20 

17 
19.5 

18 
21 

100  to 
150 

Cross  compound,  condensing,  high  speed 



30 

26 

24 

23 

23.5 

125 

Cross  compound,  non-condensing,  highspeed 

39 

31 

27 

26 

27.5 

125 

Single  cylinder  Corliss,  condensing 

120 
500 

23.7 
26.3 

20.4 
22.8 

19 
21.3 

18.5 
20.8 

19.0 
21.3 

100 
125 

Compound  Corliss,  condensing 

16.5 

14 

12.5 

12.1 

12.5 

100 

22.2 

19 

17.0 

16.5 

17.0 

150 

Single  cylinder,  rotary  four  valve,  non-con- 
densing 

75 
400 

26.2 
35.0 

22.3 
27.2 

21.3 
26.4 

21.6 
26.0 

22.8 
26.8 

100 
180 

Rotary  four  valve,  tandem  compound  non- 
condensing 

125 
600 

32.0 
40.0 

22.0 
28.3 

20 
23.2 

18.25 
22.5 

18.5 
22.7 

100 
150 

Cross    compound,    non-condensing    rotary 
four  valve 

125 
600 

25 
39.4 

21 

28 

19.1 
22.3 

18.5 
20.6 

19.0 
20.7 

100 
150 

Single    cylinder,    poppett    valve,    non-con- 
densing 

120 
600 

22.7 
28.5 

20.5 
26.0 

19.7 
25.0 

19.1 
24.3 

20.1 
25.5 

100 
150 

Single  cylinder,  poppett  valve,  condensing 

120 
600 

18.5 
24.6 

16.7 
22.3 

16.1 
21.4 

15.6 
20.8 

16.4 
21.9 

100 
150 

Compound  condensing,  poppett  valve 

200 

1200 

14.2 
18.4 

13.0 
16.9 

12.5 
16.3 

12.2 
15.9 

12.9 
16.8 

100 
150 

Uniflow 

125 
600 

14.6 
15.0 

13.7 
14.3 

13.4 
13.7 

13.4 
13.5 

13.3 
14.0 

150  . 

Steam  turbines,  condensing,  Allis-Chalmers 

300 
2000 

24 
31.9 

17 
26.3 

160 
23.8 

16.5 
23.0 

125 
175 

Steam  turbines,  condensing,  Westinghouse 

300 
2000 

13.7 
18.2 

12.8 
16.9 

12.2 
16.2 

12.6 
16.8 

125 
175 

Steam  turbines,  high  pressure,  non-con.,  12" 
to  36"  wheel,  1000  to  3600  R.P.M. 

4  to  8 
stages 



28.5 
116.5 

Ditto.     Condensing,  26-inch 

17  3 

112.0 

146  PUMPS  AND  PUMPING  STATIONS 

pistons,  the  length  of  the  stroke,  and  the  number  of  strokes  per 
unit  of  time.     A  common  form  of  this  expression  is, 


33,000' 


in  which  P  =  the  M.E.P.  in  pounds  per  square  inch; 
L  =  the  length  of  the  stroke  in  inches; 
A  =  the  sum  of  the  areas  of  the  pistons  in  square  inches; 
AT  =  the  number  of  revolutions  per  minute. 

The  I.H.P.  multiplied  by  the  mechanical  efficiency  of  the  machine 
will  give  the  brake  or  water  horse-power,  that  is,  the  horse-power 
delivered  by  the  machine.  The  product  of  the  M.E.P.,  the  sum 
of  the  areas  of  the  steam  pistons  and  the  mechanical  efficiency  of 
the  machine,  should  equal  the  product  of  the  total  head  of  water 
pumped  against  expressed  in  pounds  per  square  inch  and  the  sum 
of  the  areas  of  the  water  pistons  or  plungers.  The  M.E.P.  is 
determined  from  indicator  cards  taken  from  the  steam  cylinders 
during  operation.  These  cards  show  the  steam  pressure  on  the 
head  and  crank  ends  of  each  cylinder  at  all  points  during  the  stroke. 
81.  Steam  Turbines.  —  Among  the  advantages  in  the  use  of 
steam  turbines  as  compared  with  reciprocating  steam  engines  for 
driving  centrifugal  pumps  are  their  simplicity  of  operation,  the 
small  floor  space  needed,  their  freedom  from  vibration  requiring  a 
relatively  light  foundation,  and  their  ability  to  operate  success- 
fully and  economically  either  condensing  or  non-condensing 
under  varying  steam  pressure.  They  can  be  operated  with  steam 
at  atmospheric  or  low  pressure,  thus  taking  the  exhaust  from 
other  engines.  The  greatest  economy  of  operation  for  the  tur- 
bine alone  will  be  obtained  by  operating  with  high  pressure,  super- 
heated steam  and  with  a  vacuum  of  28  inches.  In  large  units 
the  economy  of  operation  of  steam  turbines  is  equal  to  that  of  the 
best  type  of  reciprocating  engines.  In  order  to  develop  the  high- 
est economy  turbines  are  operated  at  speeds  from  about  3,600  to 
10,000  r.p.m.  or  greater,  the  smaller  turbines  operating  at  the 
higher  speeds.  As  these  speeds  are  usually  too  great  for  the 
operation  of  centrifugal  pumps  for  lifting  sewage,  reduction  gears 
must  be  introduced  between  the  turbine  and  the  pump.  Although 
the  best  form  of  spiral-cut  reduction  gears  may  obtain  efficiencies 
of  95  to  98  per  cent,  or  even  higher,  their  use,  particularly  in  small 


STEAM  PUMPS  AND  PUMPING  ENGINES 


147 


units,  is  an  undesirable  feature  of  the  steam  turbine  for  driving 
pumps. 

The  steam  consumption  of  DeLaval  turbines  of  different 
powers,  and  the  steam  consumption  of  a  450  horse-power  DeLaval 
turbine  at  different  loads  are  shown  in  Fig.  64.  Some  steam  con- 
sumptions of  other  turbines  are  recorded  in  Table  27.  It  is  to  be 
noted  that  the  steam  consumption  of  the  450  horse-power  turbine 
at  part  loads  is  not  markedly  greater  than  that  at  full  loads. 
This  is  an  advantage  of  steam  turbines  as  compared  with  recipro- 
cating engines.  The  steam  consumption  of  any  turbine  is  depend- 
ent on  the  conditions  of  operation  and  is  lower  the  higher  the 
vacuum  into  which  the  exhaust  takes  place. 

There  are  two  types  of  turbines  in  general  use,  the  single  stage 
or  impulse  machines,  and  the  compound  or  reaction  type.  The 
DeLaval  is  a  well-known  make 
of  the  single  stage  or  impulse  type. 
The  principle  of  its  operation  is 
indicated  in  Fig.  65,  which  is  the 
trade  mark  of  the  DeLaval  Steam 
Turbine  Co.  The  energy  of  the 
steam  is  transmitted  to  the  wheel 
due  to  the  high  velocity  of  the 
steam  impinging  against  the 
vanes.  In  the  compound  or  re- 
action type  of  machine  the  steam 
expands  from  one  stage  to  the 
next  imparting  its  energy  to  the 
wheel  by  virtue  of  its  expansion 
in  the  passages  of  the  turbine. 
For  this  reason  the  single-stage 

or  impulse  type  is  operated  at  higher  speeds  than  the  compound 
or  reaction  machines. 

82.  Steam  Boilers. — Among  the  important  points  to  be  con- 
sidered in  the  selection  of  a  steam  boiler  for  a  sewage  pumping 
station  are:  the  necessary  power;  the  quality  of  the  feed  water; 
the  available  floor  space;  the  steam  pressure  to  be  carried;  and 
the  quality  and  character  of  the  fuel.  Tubular  boilers  of  the 
type  shown  in  Fig.  66,  are  lower  in  first  cost  than  other  types  of 
boilers.  They  are  not  ordinarily  built  in  units  larger  than  250  to 
300  horse-power  and  where  more  power  is  desired  a  number  of 


FIG.  65.— The  DeLaval  Trade 
Mark,  Illustrating  the  Principle 
of  the  DeLaval  Steam  Turbine. 

Courtesy,   DeLaval  Steam  Turbine  Co. 


148 


PUMPS  AND  PUMPING  STATIONS 


FIG,  66.— Horizontal  Fire-tube  Boiler. 


units  must  be  used.     They  are  objectionable  because  of  the 

relatively  large  floor  space 
required,  and  because  of  their 
relatively  poor  economy  of 
operation.  The  efficiencies 
of  water-tube  boilers  of  dif- 
ferent types  are  given  in 
Table  28.  Large  power  units 
of  the  water-tube  type,  as 
shown  in  Fig.  67,  although 
more  expensive  in  first  cost, 
require  less  floor  space.  Al- 
most any  desired  steam 
pressure  can  be  obtained 
from  either  type  but  water- 
tube  boilers  are  more  com- 
monly used  for  high  pres- 
sures. The  grate  or  stoker 

can  be  arranged  to  burn  almost  any  kind  of  fuel  under  either 
water-tube  or  fire-tube  boilers.  The  use  of  poor  quality  of  water 
in  water-tube  boilers  is  un- 
desirable as  the  tubes  are 
more  likely  to  become  clogged 
than  the  larger  passages  of 
the  fire-tube  boilers.  If  nec- 
essary, a  feed-water  puri- 
fication plant  should  be 
installed,  as  it  is  usually 
cheaper  to  take  the  inpurities 
out  of  the  water  than  to  take 
the  scale  out  of  the  boiler. 

Not  less  than  two  boiler 
units  should  be  used  in  any 
power  station,  regardless  of 
the  demands  for  power,  and 


if  the  feed  water  is  bad,  three 
or  even  four  units  should 
be  provided,  as  two  units 
may  be  down  at  any  time. 


FIG.  67. — Babcock  and  Wilcox  Water- 
tube  Boiler. 

An  appreciable  factor  of  safety  is 


provided  by  the  ability  of  a  boiler  to  be  operated  at  30  to  50  per 


STEAM  BOILERS 


149 


cent  overload,  if  sufficient  draft  is  available,  but  with  resulting 
reduction  in  the  economy  of  operation.  The  number  of  units 
provided  should  be  such  that  the  maximum  load  on  the  pumping 
station  can  be  carried  with  at  least  one  in  every  6  units  or  less, 
out  of  service  for  repairs  or  other  cause. 

TABLE  28 

EFFICIENCIES  OF  STEAM  BOILERS 
From  Marks'  Mechanical  Engineer's  Handbook 


Per 

Evap. 

Com- 

Cent 

B.T.U. 

from  and 

bined 

Sq.  Ft. 

of 

per 

at  212° 

Effi- 

Type 

" 

Furnace 

Grate 

Rated 

Lb. 

per 

ciency 

power 

Area 

Capac- 

Dry 

Lb. 

of  Boiler 

ity 

Coal 

Dry 

and 

D'vTd 

Coal 

Furnace 

Babcock  &  Wilcox 

300 

Hand-fired  

84 

118.7 

11,912 

8.81 

71.8 

Babcock  &  Wilcox 

640 

Hand-fired  

118 

121.5 

14,602 

10.83 

72.0 

Stirling 

1128 

B.  &  W.  chain  grate  . 

187 

198.3 

12,130 

9.51 

76.1 

Rust 

335 

Hand-fired  

68 

210.5 

13,202 

9.42 

68.9 

Heine 

400 

Green  chain  grate.  .  . 

83.5 

123.8 

11,608 

8.79 

73.5 

Maximum  efficient 

y  recor 

ied  

83 

The  steam  delivered  by  a  boiler  is  the  basis  of  the  measurement 
of  its  capacity  or  power.  A  boiler  horse-power  is  the  delivery  of 
33,320  B.T.U.  per  hour.  It  is  approximately  equal  to  the  raising 
of  30  pounds  of  water  per  hour  from  a  temperature  of  100° 
Fahrenheit,  to  steam  at  a  pressure  of  70  pounds  per  square  inch, 
or  to  34  pounds  of  water  per  hour  changed  to  steam  from  and  at 
212°  Fahrenheit,  at  atmospheric  pressure.  The  horse-power  of  a 
boiler  is  sometimes  approximated  by  the  area  of  its  grate  or  heat- 
ing surface.  Such  a  method  of  measuring  has  a  low  degree  of 
accuracy  on  account  of  the  variations  in  the  quality  of  the  fuel, 
and  the  rate  of  combustion.  For  example,  the  rate  of  combustion 
under  a  locomotive  boiler  is  high  and  there  is  less  than  iVth  of  a 
square  foot  of  grate  area  and  about  4.5  square  feet  of  heating 
surface  per  boiler  horse-power.  The  Scotch  Marine  type  of  boiler 
used  on  steam  ships,  has  slightly  more  grate  area  and  slightly  less 
heating  surface  than  the  locomotive  type  of  boiler,  because  the 
rate  of  combustion  is  lower.  Stationary  water-tube  boilers  may 
have  2  to  3  times  as  much  grate  area  and  heating  surface  per 


150 


PUMPS  AND  PUMPING  STATIONS 


horse-power  as  is  found  in  locomotive  boilers.  If  a  poor  type  of 
fuel  is  to  be  used  the  area  of  the  grate  should  be  increased  about 
inversely  as  the  heat  content  of  the  fuel.  The  approximate  heat 
content  of  various  types  of  fuels  is  shown  in  Table  29. 

TABLE  29 
APPROXIMATE  HEAT  VALUE  OF  FUELS 


Fuel 

B.T.U. 

per 
Pound 

Pounds  of  Water 
Evaporated  from 
and  at  212°  F. 
All  heat  utilized 

Anthracite 

13,500 

14.0 

Semi-bituminous,  Pennsylvania  

15,000 

15.5 

Semi-bituminous  best,  West  Virginia             .  . 

15,000 

15.8 

Bituminous,  best,  Pennsylvania  

14,450 

15.0 

Bituminous,  poor,  Illinois  
Lignite  best  Utah 

10,500 
11,000 

10.9 
11.4 

Lignite  poor  Oregon           .    

8,500 

8.8 

^Vood  best  oak 

9,300 

9.6 

Wood  poor  ash             

8,500 

8.8 

83.  Air  Ejectors. — The  Ansonia  compressed-air  sewage  ejector 
is  shown  in  Fig.  68.     In  its  operation,  sewage  enters  the  reservoir 
through  the  inlet  pipe  at  the  right,  the  air  displaced  being  expelled 
slowly  through  the  air  valve  marked  B.     The  rising  sewage  lifts 
the  float  which  actuates  the  balanced  piston  valve  in  the  pipe 
above  the  reservoir  when  the  reservoir  fills.     The  lifting  of  the 
valve  admits  compressed  air  to  the  reservoir.     The  air  pressure 
closes  valve  A  and  the  inlet  valve  at  the  right,  and  ejects  the 
sewage  through  the  discharge  pipe  at  the  left.     As  the  float  drops 
with  the  descending  sewage  it  shuts  off  the  air  supply  and  opens 
the  air  exhaust  through  the  small  pipe  at  the  top  center.     Sewage 
is  prevented  from  flowing  back  into  the  reservoir  by  the  check 
valve  in  the  discharge  pipe.     Other  ejectors  operating  on  a  similar 
principle  are  the  Ellis,  the  Pacific,  the  Priestmann  and  the  Shone. 

84.  Electric  Motors. — The  most  common  form  of  alternating 
current  electric  motor  used  for  driving  sewage  pumps  where  con- 
tinuous operation  and  steady  loads  are  met  is  the  squirrel-cage 
polyphase  induction  motor.     These  motors  operate  at  a  nearly 


ELECTRIC   MOTORS 


151 


constant  speed  which  should  be  selected  to  develop  the  maximum 
efficiency  of  the  pump  and  motor  set.  While  Fig.  59  shows  the 
best  efficiency  under  varying  heads  to  be  obtained  with  variable 
speed,  the  advantages  of  cost,  attention,  and  availability  make 
the  use  of  a  constant  speed  motor  common.1  This  type  of  motor 
is  undesirable  where  stopping  and  starting  are  frequent  because 
it  has  a  relatively  small  starting  torque  and  it  requires  a  large 


FIG.  68. — Ansonia  Compressed-Air  Sewage  Ejector. 

starting  current.  Such  motors  can  be  constructed  in  small  sizes 
for  high  starting  torques  by  increasing  the  resistance  of  the  rotor, 
but  at  the  expense  of  the  efficiency  of  operation. 

Alternating  current  motors  are  more  generally  used  than  direct- 
current  motors  because  of  the  greater  economy  of  transmission  of 
alternating  current,  but  where  direct  current  is  available  constant 
speed  shunt  wound  motors  should  be  adopted. 

1  "  The  Economy  Resulting  from  the  Use  of  Variable  Speed  Induction 
Motors  for  Driving  Centrifugal  Pumps  "  by  M.  L.  Enger  and  W.  J.  Putnam. 
Journal  Am.  Water  Works  Ass'n.,  1920,  Vol.  7,  p.  536. 


152  PUMPS  AND  PUMPING  STATIONS 

In  the  selection  of  a  motor  to  drive  a  centrifugal  pump  it  is 
important  that  the  motor  have  not  only  the  requisite  power,  but 
that  its  speed  will  develop  the  maximum  efficiency  from  the  pump 
and  motor  combined.  If  the  pump  and  motor  operate  on  the 
same  shaft  the  speed  of  the  two  machines  must  be  the  same.  If 
the  two  are  belt  connected,  the  size  of  the  pulleys  may  be  selected 
so  as  to  give  the  required  speed.  If  the  motor  is  to  be  connected 
to  a  power  pump  an  adequate  automatic  pressure  relief  valve 
should  be  provided  on  the  discharge  pipe  from  the  pump,  to  pre- 
vent the  overloading  of  the  motor  or  bursting  of  the  pump  in  case 
of  a  sudden  stoppage  in  the  pipe.  The  motor  must  be  selected  to 
suit  the  conditions  of  voltage,  cycle,  and  phase  on  the  line.  Trans- 
formers are  available  to  step  the  voltage  up  or  down  to  practically 
any  value.  Rotary  converters  are  used  to  change  direct  to  alter- 
nating current  or  vice  versa. 

85.  Internal  Combustion  Engines.  —  Internal  combustion 
engines  are  used  for  driving  pumps.  Units  are  available  in  size 
from  fractions  of  1  horse-power  to  2,000  horse-power  or  more, 
although  the  use  of  the  larger  sizes  is  exceptional.  These  engines 
are  not  commonly  used  for  sewage  pumping  but  when  used  they 
are  ordinarily  belt  connected  to  a  centrifugal  pump,  or  to  an 
electric  generator  which  in  turn  drives  electric  motors  which 
operate  centrifugal  pumps.  This  type  of  engine  is  more  com- 
monly adapted  to  small  loads,  although  not  entirely  confined  to 
this  field,  as  they  serve  admirably  as  emergency  units  to  supple- 
ment an  electrically  equipped  pumping  station.  The  fuel  effi- 
ciency of  internal  combustion  engines  is  higher  than  for  steam 
engines  as  is  indicated  in  Table  30,  but  the  fuel  is  more  expensive. 

The  four-cycle  gas  engine  shown  in  Fig.  69  is  the  type  most 
commonly  used.  Its  horse-power  is  the  product  of:  the  mean 
effective  pressure,  the  length  of  the  stroke,  the  area  of  the  piston, 
and  the  number  of  explosions  per  second  divided  by  550.  The 
M.E.P.  is  dependent  on  the  character  of  the  fuel  used  and  the 
compression  of  the  gas  before  ignition.  Producer  gas  will  furnish 
mean  effective  pressures  between  60  and  70  pounds  per  square 
inch,  natural  gas  and  gasoline,  85  to  90  pounds  per  square  inch, 
and  alcohol  from  95  to  110  pounds  per  square  inch. 

The  Diesel  Engine  is  the  most  efficient  of  internal  combustion 
engines.  The  original  aim  of  the  inventor,  Dr.  Rudolph  Diesel, 
was  to  avoid  the  explosive  effect  of  the  ordinary  internal  com- 


INTERNAL  COMBUSTION  ENGINES 


153 


TABLE  30 
COMPARATIVE  FUEL  COSTS  FOR  PRIME  MOVERS 


Type  of  Engine 

Quantity  of  Fuel 
per  H.P.  Hour 

Cost  of  Fuel 
in  Cents  per 
Horse-power 
Hour 

Reciprocating  steam  engines,  simple,  non- 
condensing  25  to  200  H  P 

21  to    8  Ib  coal 

4  2  to    16 

Triple  condensing,  2000  to  10,000  H.P.  . 

2.  3  to  1.91b.  coal 

0.46  to  0.37 

Steam  turbines,   high  pressure,   non-con- 
densing, 
200  to  500  K  W 

6  5  to  4  2  Ib  coal 

13    to  0  86 

500  to  3000  K.W  

2.6  to  1.91b.  coal 

0.52  to  0.37 

Condensing  5000  to  20,000  K.W  

1.8tol.431b.  coal 

0.36  to  0.28 

Gas  engines 
Natural  gas,  50  to  200  H.P  

19  to  11  cu.  ft. 

Producer  gas,  50  to  200  H  P 

2  to  1  5  eu  ft 

Illuminating  gas,  10  to  75  H.P  

26  to  19  cu.  ft. 

2.1to    1.5 

Gasoline,  10  to  75  H  P     

1  5  to  0  8  pints 

5  6to    3  0 

Oil  engines,  100  to  500  H.P  

I.lto0.751b.oil 

NOTE. — Coal   assumed  at  $4.00  per  ton,  illuminating  gas   at  80  cents   per    thousand 
cubic  feet,  and  gasoline  at  30  cents  per  gallon. 


FIG.  69.— Bessemer  Oil  Engim 
Twin  Cylinder,  Valve  Side. 


154  PUMPS  AND  PUMPING  STATIONS 

bustion  engine  by  injecting  a  fuel  into  air  so  highly  compressed 
that  its  heat  would  ignite  the  fuel,  causing  slow  combustion  of 
the  fuel  thus  utilizing  its  energy  to  a  greater  extent.  The  fuel 
and  air  were  to  be  so  proportioned  as  to  require  no  cooling. 
Although  the  ideal  condition  has  not  been  attained,  the  heat 
efficiency  of  Diesel  engines  is  high.  They  will  consume  from 
0.3  to  0.5  of  a  pound  of  oil  (containing  18,000  B.T.U.  per  pound) 
per  brake  horse-power  hour,  giving  an  effective  heat  efficiency  of 
25  to  30  per  cent.  Although  not  now  in  extensive  use  in  the 
United  States  it  is  probable  that  this  engine  will  be  more  generally 
adopted  for  conditions  suitable  for  internal  combustion  engines. 

86.  Selection  of  Pumping  Machinery. — Centrifugal  pumps 
are  particularly  adapted  to  the  lifting  of  sewage  because  of  their 
large  passages,  and  their  lack  of  valves.  The  low  lifts,  nearly 
constant  head,  and  the  possibility  of  equalizing  the  load  by 
means  of  reservoirs  are  particularly  suited  to  efficient  operation 
of  centrifugal  pumps.  They  require  less  floor  space  than  recipro- 
cating pumps  of  the  same  capacity,  and  because  of  their  freedom 
from  vibration  they  do  not  demand  so  heavy  a  foundation.  The 
discharge  from  the  pump  is  continuous  thus  relieving  the  piping 
from  vibration.  In  case  of  emergency  the  discharge  valve  can 
be  shut  off  without  shutting  down  the  pump,  an  important  point 
in  "  fool  proof  "  operation. 

Volute  pumps  are  better  adapted  to  pumping  sewage  as  their 
passages  are  more  free  and  they  are  better  suited  to  the  low  lifts 
met.  Gritty  and  solid  matter  will  cause  wear  on  the  diffusion 
vanes  of  turbine  pumps  in  spite  of  the  most  careful  design. 
Although  turbine  pumps  can  possibly  be  built  with  higher  effi- 
ciency than  volute  pumps,  their  efficiency  at  part  load  falls  rapidly 
and  the  fluctuations  of  sewage  flow  are  sufficient  to  affect  the 
economy  of  operation.  Turbine  pumps  are  more  expensive  and 
heavier  than  volute  pumps  on  account  of  the  increased  size  neces- 
sitated by  the  diffusion  vanes. 

Multi-stage  pumps  are  used  for  high  lifts  and  are  seldom  if 
ever  required  in  sewage  pumping.  As  ordinarily  manufactured, 
each  stage  is  good  for  an  additional  40  to  100  pounds  pressure, 
but  wide  variations  in  the  limiting  pressures  between  stages  are 
to  be  found. 

Reciprocating  plunger  pumps  are  sometimes  used  for  sewage 
pumping  where  the  character  of  the  sewage  is  such  that  the 


SELECTION  OF  PUMPING  MACHINERY  155 

valves  will  not  be  clogged  nor  parts  of  the  pump  corroded.  These 
pumps  are  seldom  used  in  small  installations  or  for  low  lifts. 
They  are  not  adapted  to  automatic  or  long  distance  control  as 
are  electrically  driven  centrifugal  pumps.  The  use  of  recipro- 
cating pumps  for  sewage  pumping  is  practically  restricted  to  very 
large  pumping  stations  with  capacities  in  the  neighborhood  of 
50,000,000  gallons  per  day  or  more.  Steam-driven  pumps  are 
the  most  common  of  the  reciprocating  type,  but  power  pumps  are 
sometimes  used  in  special  cases  for  small  installations  and  may  be 
driven  by  either  a  steam  or  gas  engine  or  an  electric  motor. 

Compressed  air  ejectors,  as  described  in  Art.  83  are  used  for 
lifting  sewage  and  other  drainage  from  the  basement  of  buildings 
below  the  sewer  level. 

Centrifugal  pumps  electrically  driven  are,  as  a  rule,  the  most 
satisfactory  for  sewage  pumping.  Electric  drive  lends  itself  to 
control  by  automatic  devices,  which  are  particularly  convenient 
in  small  pumping  stations.  The  control  can  be  arranged  so  that 
the  pump  is  operated  only  at  full  load  and  high  efficiency,  and 
when  not  operating  no  power  is  being  consumed,  as  is  not  the 
case  with  a  steam  pump  where  steam  pressure  must  be  maintained 
at  all  times.  The  electric  driven  pump  is  thrown  into  operation 
by  a  float  controlled  switch  which  is  closed  when  the  reservoir 
fills,  and  opens  when  the  pump  has  emptied  the  reservoir.  The 
choice  between  steam  and  electric  power  for  large  pumping  stations 
is  a  matter  of  relative  reliability  and  economy. 

The  selection  of  the  proper  type  of  pump,  whether  recipro- 
cating or  otherwise,  requires  some  experience  in  the  consideration 
of  the  factors  involved.  Fig.  70  is  of  some  assistance.  In  dis- 
cussing this  figure,  Chester  states: 

"  Fig.  70  attempts  to  represent  graphically,  the  writer's 
ideas  under  general  conditions,  of  the  machines  that  should 
be  selected  for  certain  capacities  for  both  principal  engine 
and  alternate  and  the  station  duty  they  may  be  expected 
to  produce,  but  you  must  realize  that  this  intends  the 
principal  engine  doing  at  least  90  per  cent  of  the  work  and 
that  the  head,  the  cost  of  coal,  the  load  factor,  the  cost 
of  real  estate  .  .  .  the  boiler  pressure,  and  the  space  avail- 
able, and  finally  .  .  .  the  funds  available,  are  factors  which 
may  shift  both  the  horizontal  and  curved  lines.  In  the 
field  of  low  service  pumps  of  10,000,000  capacity  or  over, 
the  centrifugal  pump  reigns  supreme,  and  for  constant 


156  PUMPS  AND  PUMPING  STATIONS 

low  heads  of  20,000,000  capacity  or  over  the  turbine  driven 
centrifugal  usurps  the  field." 

A  reciprocating  pump  of  any  type  would  have  to  be  specially 
built  for  pumping  sewage  not  carefully  screened  or  otherwise 
treated,  as  the  valves,  ordinarily  used  in  such  pumps  for  lifting 
water,  would  clog.  The  vertical  triple-expansion  pumping 
engine  with  special  valves  and  for  large  installations,  and  the 
centrifugal  pump  for  large  or  small  installations  are  the  only  suit- 


Qirecf-Acting  Triple  Expansion,/ 


0     I     2     3    456     7    8    9    10    II     \l   13    14    15   16    17    18    19  20 
Capacity   in    Millions  of  Gallons  per  Day. 

FIG.  70. — Expectancy  Curves  for  Pumping  Engines  Working  against  a  Pres- 
sure of  100  Pounds  per  Square  Inch. 

J.  N.  Chester,  Journal  Am,  Water  Works  Ass'n,  Vol.  3,  1916,  p.  493. 

able  types  for  pumping  sewage.     With  steam  turbine  or  electric 
drive  the  centrifugal  has  the  field  to  itself. 

87.  Costs  of  Pumping  Machinery. — The  cost  of  pumping 
machinery  can  not  be  stated  accurately  as  the  many  factors 
involved  vary  with  the  fluctuations  in  the  prices  of  raw  materials, 
transportation,  labor,  etc.  The  actual  purchase  price  of  machinery 
can  be  found  accurately  only  from  the  seller.  The  costs  given  in 
this  chapter  are  useful  principally  for  comparative  purposes  and 
for  exercise  in  the  making  of  estimates.  The  costs  of  complete 
pumping  stations  are  shown  in  Table  3 1.1  These  figures  repre- 
sent costs  in  1911.. 

1  C.  A.  Hague  in  Trans.  Am.  Society  of  Civil  Engineers,  Vol.  74,  1911, 
p.  20. 


COST  COMPARISONS  OF  DIFFERENT  DESIGNS 


157 


TABLE  31 

COSTS  OF  COMPLETE  PUMPING  STATIONS 

These  costs  include  the  best  type  of  triple  expansion  engines,  high-pressure 
boilers,  brick  or  inexpensive  stone  building  with  slate  roof,  chimney  and  intake. 
Cost  of  land  is  not  included. 


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438 

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32 

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8,000 

120 

48 

187 

9,000 

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362 

7,250 

90 

36 

229 

8,250 

130 

52 

192 

10,000 

60 

24 

312 

7,500 

100 

40 

213 

8,500 

88.  Cost  Comparisons  of  Different  Designs.  —  In  the  design  of 
a  pumping  station  and  its  equipment  the  relative  costs  of  different 
designs  should  be  compared,  and  the  least  expensive  design 
selected,  due  consideration  being  given  to  serviceability,  reliability, 
and  other  factors  without  definite  financial  value.  In  comparing 
the  costs  of  different  types  of  machinery,  all  items  in  connection 
with  the  pumping  station  should  be  considered.  For  example, 
the  cost  of  an  electrically  driven  centrifugal  pump  and  equipment 
may  be  less  than  the  total  cost  of  a  steam  driven  reciprocating 
pump  and  equipment  because  of  the  saving  in  the  cost  of  boilers, 
boiler  house,  etc.,  but  a  comparison  of  the  capitalized  cost  of  the 
two  might  show  in  favor  of  the  reciprocating  steam  pump  because 
of  the  lower  cost  of  operation. 

The  total  cost  of  a  plant,  or  any  portion  thereof,  may  be 
considered  as  made  up  of  three  parts  :  (1)  The  first  cost,  (2)  opera- 
tion and  maintenance  and,  (3)  renewal.  The  total  cost  S  can  be 
expressed  as 


in  which 


C  =  the  first  cost; 

0  =  the  annual  expenditure  for  operation  and  mainte- 

nance; 

#  =  the  amount  set  aside  to  cover  renewal; 
r  =  the  rate  of  interest. 


158  PUMPS  AND  PUMPING  STATIONS 

S  is  called  the  capitalized  cost  of  a  plant.     The  annual  payment 
necessary  to  perpetuate  a  plant  is 


The  value  of  R  is  useful  when  expressed  in  terms  of  the  life  of  the 
plant  or  machine  and  the  current  rate  of  interest.  It  is  sometimes 
called  the  depreciation  factor  or  capitalized  depreciation.  If  it 
is  borne  in  mind  that  R  is  the  amount  to  be  set  aside  at  compound 
interest  for  the  life  of  the  plant,  at  the  end  of  which  time  the 
accrued  interest  should  be  sufficient  to  renew  the  plant,  it  is  evi- 

dent that 

R(l+R)n-R  =  C 

C 
*=(!+,.)._! 

in  which  n  is  the  period  of  usefulness,  or  life  of  the  plant,  expressed 
in  years,  no  allowance  being  made  for  scrap  value. 

A  comparison  of  the  annual  expense  of  three  different  plants  is 
shown  in  Table  32.  It  is  evident  from  this  comparison  that  the 
machinery  with  the  least  first  cost  is  not  always  the  least  expen- 
sive when  all  items  are  considered. 

A  sinking  fund  is  a  sum  of  money  to  which  additions  are  made 
annually  for  the  purpose  of  renewing  a  plant  at  the  expiration  of 
its  period  of  usefulness.  The  annual  payment  into  the  sinking 
fund  is  equivalent  to  the  term  Rr  in  the  expression  for  annual 
cost,  or  in  terms  of  C,  r,  and  n,  the  annual  payment  is 

Cr 


It  is  the  same  as  the  capitalized  depreciation  multiplied  by  the 
rate  of  interest.     The  expression  ,..  ,    \n_i  *s  sometimes  called 

the  rate  of  depreciation. 

The  present  worth  of  a  machine  is  the  difference  between  its 
first  cost  and  the  present  value  of  the  sinking  fund.  If  m  repre- 
sents the  present  age  of  a  plant  in  years,  then  the  present  worth  is 

p=c(i- 


Where  straight-line  depreciation  is  spoken  of  it  is  assumed  that 
the  worth  of  a  machine  depreciates  an  equal  part  of  its  first  cost 


COST  COMPARISONS  OF  DIFFERENT  DESIGNS 


159 


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160 


PUMPS  AND  PUMPING  STATIONS 


each  year.  For  example,  if  the  life  of  a  plant  is  assumed  to  be 
20  years,  straight-line  depreciation  will  assume  that  the  plant 
loses  2^  °f  its  original  value  annually.  The  present  worth  of  a 
plant  under  this  assumption  would  be  the  product  of  its  first  cost 
and  the  ratio  between  its  remaining  life  and  its  total  life.  This 
method  of  estimating  depreciation  and  worth  is  frequently  used, 
particularly  for  short-lived  plants  and  for  simplicity  in  book- 
keeping, but  it  is  less  logical  than  the  method  given  above. 

89.  Number  and  Capacity  of  Pumping  Units. — In  order  to 
select  the  number  and  capacity  of  pumping  units  for  the  best 
economy,  a  comparison  of  the  costs  of  different  combinations  of 
units  should  be  made  and  the  most  economical  combination 
determined  by  trial.  The  principles  outlined  in  the  preceding 
articles  should  be  observed  in  making  these  comparisons.  In  a 
steam  pumping  station,  when  the  number  of  units  operating  is 
less  than  the  average  daily  maximum  for  the  period,  steam  must 
nevertheless  be  kept  on  a  sufficient  number  of  boilers  to  operate 
the  maximum  number  of  pumps.  This,  and  corresponding 
standby  losses  must  not  be  overlooked,  as  they  may  show  that  a 
smaller  number  of  larger  units  is  ultimately  more  economical. 

TABLE  33 

SUMMARY  OF  FLUCTUATIONS   OF  SEWAGE   FLOW   AT  A  PROPOSED 
PUMPING  STATION 


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Total  horse-power  days  for  one  year,  102,000. 
Average  load  in  horse-power,  280. 

For  example,  the  sewage  flow  expected  at  a  proposed  pumping 
station  is  shown  in  Table  33.     The  steps  involved  in  the  selection 


NUMBER  AND  CAPACITY  OF  PUMPING  UNITS 


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PUMPS  AND  PUMPING  STATIONS 


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NUMBER  AND  CAPACITY  OF  PUMPING  UNITS         163 

of  the  number  and  capacity  of  pumping  units  to  care  for  these 
quantities  are  as  follows:  (1)  Determine  the  rated  capacity  of 
the  equipment  to  be  provided.  In  this  case  the  capacity  will  be 
taken  as  450  horse-power,  which  is  the  maximum  load  to  be  placed 
on  the  pumps.  (2)  Select  any  number  of  units  of  such  different 
types  and  capacities  as  are  available  for  comparison,  and  arrange 
them  in  different  combinations  so  that  each  unit  will  operate  as 
nearly  as  possible  at  its  rated  capacity.  The  work  involved  in 
such  a  study  for  5  units  is  shown  in  Table  34.  The  weight  of 
steam  consumed  per  indicated  horse-power  hour  corresponding 
to  the  per  cent  of  the  rated  capacity  at  which  the  unit  is  operating 
is  read  from  Fig.  64  or  other  data.  (3)  Repeat  this  step  for  other 
numbers  and  types  of  units.  (4)  Prepare  a  table  showing  the 
annual  costs  of  combinations  of  different  numbers  and  types  of 
units  as  shown  for  this  example  in  Table  35.  The  figures  in  Table 
35  show  that  the  least  expensive  of  the  combinations  of  the  units 
studied  is  one  200  horse-power  unit,  and  one  250  horse-power 
unit,  with  a  250  horse-power  unit  in  reserve.  It  is  to  be  noted 
that  a  reserve  unit  has  been  provided  in  each  combination,  the 
capacity  of  which  is  equal  to  that  of  the  largest  unit  of  the  com- 
bination. 


CHAPTER  VIII 
MATERIALS  FOR  SEWERS 

90.  Materials. — The  materials  most  commonly  used  for  the 
manufacture  of  sewer  pipe  are  vitrified  clay  and  concrete.  Cast 
iron,  steel,  and  wood  are  also  used,  but  only  under  special  condi- 
tions. For  pipes  built  in  the  trench,  concrete,  concrete  blocks, 
brick,  and  vitrified  clay  blocks  are  used.  Concrete  is  being  used 
to-day  more  than  bricks  or  blocks  because  it  is  cheaper.  A  decade 
or  more  ago  all  large  sewers  were  built  of  bricks.  Vitrified  clay 
and  concrete  are  used  for  manufactured  pipe  42  inches  and  less  in 
diameter.  Concrete  is  used  almost  exclusively  for  larger  sizes  of 
pipe,  particularly  for  pipe  constructed  in  place,  although  a  brick 
invert  lining  is  advisable  when  high  velocities  of  flow  are  expected. 

The  character  of  the  external  load,  the  velocity  of  flow  and  the 
quality  of  sewage  are  important  factors  in  determining  the  material 
to  be  used  in  the  construction  of  sewers.  Reinforced  concrete 
should  be  used  for  large  sewers  near  the  surface  subjected  to 
heavy  moving  loads.  A  high  velocity  of  flow  with  erosive  sus- 
pended matter  demand  a  brick  wearing  surface  on  the  invert. 
Many  engineers  consider  concrete  less  suitable  than  vitrified  clay 
or  brick  for  conveying  septic  sewage  or  acid  industrial  wastes,  as 
concrete  deteriorates  more  rapidly  under  such  conditions.  Con- 
crete should  be  used  on  soft  yielding  foundations,  whereas  a  hard 
compact  earth,  which  can  be  cut  to  the  form  of  the  sewer,  is  suit- 
able to  the  use  of  brick  or  concrete. 

Cast-iron  pipe  with  lead  joints  is  used  for  sewers  flowing  under 
pressure,  or  where  movements  of  the  soil  are  to  be  expected.  If 
the  sewage  is  not  flowing  under  pressure,  cement  joints  are  some- 
times used  in  the  cast-iron  pipe.  Movements  of  the  soil  are  to 
be  expected  on  side  hills,  under  railroad  tracks,  etc.  Steel  pipe 
is  used  on  long  outfalls  or  under  other  conditions  where  external 
loads  are  light  and  the  cost  is  less  than  for  other  materials. 
Because  of  the  thin  plates  used  and  the  liability  to  corrosion  steel 
is  not  frequently  used.  It  should  never  be  deeply  buried  nor 

164 


VITRIFIED  CLAY  PIPE 


165 


externally  loaded  because  of  its  weakness  in  resisting  such  forces. 
Like  wood  pipe,  its  lightness  is  favorable  to  use  on  bridges,  but  the 
greater  heat  conductivity  of  steel  than  wood  necessitates  protection 
against  freezing  in  exposed  positions.  Wood  is  preferable  only  where 
the  economy  of  its  use  is  pronounced  and  the  pipe  is  running  full 
at  all  times.  It  is  desirable  that  the  wood  pipe  should  be  always 
submerged  as  the  life  of  alternately  wet  and  dry  wood  is  short. 

Corrugated  galvanized  iron  and  unglazed  tile  have  been  used  for 
sewers,  but  usually  only  in  emergencies  or  as  a  make-shift.  Corru- 
gated iron  is  not  suitable  on  account  of  its  roughness  and  liability 
to  corrosion,  and  unglazed  tile  because  of  its  lack  of  strength. 

91.  Vitrified  Clay  Pipe. — In  general  the  physical  and  chemical 
qualities  of  clays  before  burning  are  not  sufficient  to  cause  their 
condemnation  or  approval  by 
the  engineer,  as  their  behavior  in 
the  furnace  is  quite  individual 
and  depends  greatly  on  the  man- 
ner in  which  they  are  fired.  The 
engineer  is  interested  in  the  re- 
sult and  writes  his  specifications 
accordingly. 

In  the  manufacture  of  clay 
pipe,  the  clay  as  excavated  is  taken 
to  a  mill  and  ground  while  dry,  to 
as  fine  a  condition  as  possible.  It 
is  then  sent  to  storage  bins  from 
which  it  is  taken  for  wet  grind- 
ing and  tempering.  In  this  proc- 
ess the  clay  is  mixed  with  water 
to  the  proper  degree  of  plasti- 
ity.  A  variation  of  1  to  1J  per 
cent  in  the  moisture  content  will 
mean  failure.  Too  wet  a  mix- 
ture will  not  have  sufficient 
strength  to  maintain  its  shape 
in  the  kiln.  Too  dry  a  mixture 
will  show  laminations  as  it  is 
pressed  through  the  discs. 

A  press  used  in  the  manufacture  of  clay  pipe  is  shown  in 
cross-section  in  Fig.  71.     With  the  piston  heads  in  the  steam  and 


h. 


Hub 
Former 

,-h 


FIG.  71. — Diagrammatic  Section 
through  Clay-pipe  Press. 


166 


MATERIALS  FOR  SEWERS 


mud  cylinders  at  their  extreme  upward  positions,  the  mud  cylinder 
is  filled  with  clay  of  the  proper  consistency.  Steam  is  then  turned 
into  the  steam  cylinder  under  pressure  and  the  clay  is  squeezed 
into  the  space  between  the  inner  and  outer  shells  of  the  die  and 
mandrel  to  form  the  hub  of  the  pipe.  The  pressure  on  the  clay 
may  be  from  250  to  600  pounds  per  square  inch.  When  clay 
appears  at  the  holes,  marked  hh  at  the  bottom  of  the  mud  cylinder, 


PIG.  72. — Clay-pipe  Press. 

Courtesy,  Blackmer  and  Post  Manufacturing  Co. 

the  bottom  plate  and  the  center  portion  of  the  die  are  removed 
and  the  remainder  or  straight  portion  of  the  pipe  is  formed  by 
squeezing  the  clay  between  the  mandrel  and  the  outer  wall  of  the 
die.  A  completely  formed  pipe  can  be  seen  issuing  from  the  press 
in  Fig.  72.  Any  sized  pipe  that  is  desired  can  be  formed  from  the 
same  press  by  changing  the  size  of  the  dies  and  mandrel. 

Curved  pipes  are  made  in  two  ways — by  bending  directly  as 
they  issue  from  the  press,  or  by  shaping  by  hand  in  plaster  of 
paris  molds.  Junctions  are  made  by  cutting  the  branch  pipe  to 
the  shape  of  the  outside  of  the  main  pipe,  fastening  the  branch 


VITRIFIED  CLAY  PIPE  167 

in  place  with  soft  clay  and  then  cutting  out  the  wall  of  the  main 
pipe  the  size  of  the  branch.  Special  fittings  are  usually  made  by 
hand  in  plaster  molds. 

After  being  pressed  into  shape  the  pipes  are  taken  to  a  steam- 
heated  drying-room  where  a  constant  temperature  is  maintained 
in  order  to  prevent  cracking  of  the  pipes.  They  remain  in  the 
drying  room  from  3  to  10  days  until  diy,  when  they  are  taken  to 
the  kilns.  If  taken  to  the  kilns  when  moist  blisters  will  be  pro- 
duced. 

The  dried  pipes  are  piled  carefully  in  the  kiln  so  that  heat  and 
weight  may  be  as  evenly  distributed  as  possible,  and  the  fire  is 
then  started  in  the  kiln.  The  process  of  burning  can  be  roughly 
divided  into  five  stages: 

1st.  Water  smoking,  which  lasts  about  72  hours  during  which 
the  temperature  is  raised  gradually  to  350  degrees  Fahrenheit. 

2nd.  Heating,  during  which  the  temperature  is  raised  to  800 
degrees  Fahrenheit  in  24  hours. 

3rd.  Oxidation,  during  which  the  temperature  is  raised  to 
1,400  degrees  Fahrenheit  in  84  hours. 

4th.  Vitrification,  in  which  the  temperature  is  raised  to  2,100 
degrees  Fahrenheit  in  48  hours,  and  finally, 

5th.  Glazing,  during  which  the  temperature  is  unchanged  but 
salt  (NaCl)  is  thrown  in  and  allowed  to  burn. 

Oxidation  must  be  complete  before  vitrification  is  started  as 
otherwise  blisters  will  be  raised  due  to  imprisoned  carbon  dioxide. 
The  important  points  in  vitrification  are  to  make  the  required 
temperature  within  a  reasonable  time  and  to  maintain  a  uniform 
distribution  of  heat  throughout  the  kiln.  When  vitrification  is 
complete  as  shown  by  a  glassy  fracture  of  a  broken  sample  taken 
from  the  kiln,  glazing  is  accomplished  by  throwing  a  shovelful  of 
salt  on  the  hottest  part  of  the  fire.  About  five  to  six  applications 
of  salt  from  two  to  three  hours  apart  may  be  needed.  The  kiln 
is  then  allowed  to  cool  and  the  manufacture  of  the  pipe  is  com- 
plete. The  completeness  of  vitrification  is  indicated  by  the 
amount  of  water  that  the  finished  pipe  will  absorb.  Completely 
vitrified  pipe  will  absorb  no  moisture.  Soft-burned  pipe  may 
absorb  as  much  as  15  per  cent  moisture. 

Vitrified  clay  blocks  are  made  of  the  same  material  and  in  the 
same  manner  as  vitrified  clay  pipe. 

The  following  data  on  vitrified  pipe  have  been  abstracted  from 


168  MATERIALS  FOR  SEWERS 

the  specifications  for  vitrified  pipe  adopted  by  the  American 
Society  for  Testing  Materials. 

Pipes  shall  be  subject  to  rejection  on  account  of  the  following: 

(a)  Variation  in  any  dimension  exceeding  the  per- 
missible variations  given  in  Table  36. 

(6)  Fracture  or  cracks  passing  through  the  shell  or 
hub,  except  that  a  single  crack  at  either  end  of  a  pipe  not 
exceeding  2  inches  in  length  or  a  single  fracture  in  the  hub 
not  exceeding  3  inches  in  width  nor  2  inches  in  length  will 
not  be  deemed  cause  for  rejection  unless  these  defects 
exist  in  more  than  5  per  cent  of  the  entire  shipment  or 
delivery. 

(c)  Blisters  or  where  the  glazing  is  broken  or  which 
exceed  3  inches  in  diameter,  or  which  project  more  than 
|  inch  above  the  surface. 

(d)  Laminations  which  indicate  extended  voids  in  the 
pipe  material. 

(e)  Fire  cracks  or  hair  cracks  sufficient  to  impair  the 
strength,  durability  or  serviceability  of  the  pipe. 

(/)  Variations  of  more  than  i  inch  per  linear  foot  in 
alignment  of  a  pipe  intended  to  be  straight. 

(g)  Glaze  which  does  not  fully  cover  and  protect  all 
parts  of  the  shell  and  ends  except  those  exempted  in  Sect. 
31.  Also  glaze  which  is  not  equal  to  best  salt  glaze. 

(h)  Failure  to  give  a  clear  ringing  sound  when  placed 
on  end  and  dry  tapped  with  a  light  hammer. 

(i)   Insecure  attachment  of  branches  or  spurs. 

Workmanship  and  Finish 

(29)  Pipes  shall  be  substantially  free  from  fractures, 
large  or  deep  cracks  and  blisters,  laminations  and  surface 
roughness. 

(31)  The  glaze  shall  consist  of  a  continuous  layer  of 
bright  or  semi-bright  glass  substantially  free  from  coarse 
blisters  and  pimples.     .     .     .     Not  more  than  10  per  cent 
of  the  inner  surface  of  any  pipe  barrel  shall  be  bare  of 
glaze  except  the  hub,  where  it  may  be  entirely  absent. 
Glazing  will  not  be  required  on  the  outer  surface  of  the 
barrel  at  the  spigot  end  for  a  distance  from  the  end  equal 
to  f  the  specified  depth  of  the  socket  for  the  corresponding 
size  of  pipe.     Where  glazing  is  required  there  shall  be 
absence  of  any  well  defined  network  of  crazing  lines  or 
hair  cracks. 

(32)  The  ends  of  the  pipe  shall  be  square  with  their 
longitudinal  axis. 

(33)  Special  shapes  shall  have  a  plain  spigot  end  and 


VITRIFIED   CLAY  PIPE 


169 


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170  MATERIALS  FOR  SEWERS 

a  hub  end  corresponding  in  all  respects  with  the  dimensions 
specified  for  pipes  of  the  corresponding  internal  diameter. 

(a)  Slants  shall  have  their  spigot  ends  cut  at  an  angle 
of  approximately  45  degrees  with  the  longitudinal  axis. 


Decrease?  1-8  Curve  Increaser 


P  Trap  without      Running  Trap  with         P  Trap  with 
Hand  Hole  Hand  Hole  Hand  Hole 


Double  T  Branch    Running  Trap  with- 
out Hand  Hole 


Breeches 


Running  Trap  with    Doable  Y  Branch 
•    Side  Hand  Hole 

FIG.  73.— Standard  Clay  Pipe  Specials. 

Courtesy,  Blackmer  and  Post  Manufacturing  Co. 

(6)  Curves  shall  be  at  angles  of  90,  45,  22J,  and  11J 
degrees  as  required.  They  shall  conform  substantially  to 
the  curvature  specified. 

(c)     .    .     .    All  branches  shall  terminate  in  sockets. 

In  Fig.  73  are  shown  the  various  forms  of  vitrified  pipe  and 
specials  which  are  ordinarily  available  on  the  market. 


VITRIFIED  CLAY  PIPE 


171 


The  life  of  vitrified  clay  sewers  and  some  observations  on  the 
results  of  the  inspection  of  the  sewers  in  Manhattan  are  discussed 
in  Chapter  XII.  The  strength  of  vitrified  sewer  pipes  is  shown 
in  Table  37. 


TABLE  37 

STRENGTH  OF  SEWER  PIPE 

Strength  in  pounds  per  linear  foot  to  carry  loads  from  ditch  filling  material 
such  as  ordinary  sand  and  thoroughly  wet  clay,  with  the  under  side  of  the 
pipe  bedded  60°  to  90°  by  ordinary  good  methods.  From  Proc.  Am.  Society 
for  Testing  Materials,  Vol.  20,  1920,  page  604. 


Height 
of  Fill 
Above 


Breadth  of  the  Ditch  a  Little  Below  the  Top  of  the  Pipe 

1  Foot 

2  Feet 

3  Feet 

4  Feet 

5  Feet 

lop  01 
Pipe, 
Feet 

Ditch  Filling  Material 

sand 

clay 

sand 

clay 

sand 

clay 

sand 

clay 

sand 

clay 

2 

265 

280 

615 

635 

970 

990 

1330 

1,350 

1,690 

1,710 

4 

400 

450 

1055 

1125 

1745 

1825 

2455 

2,535 

3,165 

3,250 

6 

470 

545 

1370 

1500 

2370 

2525 

3405 

3,575 

4,460 

4,740 

8 

505 

605 

1600 

1790 

2875 

3115 

4215 

4,495 

5,595 

5,890 

10 

525 

640 

1765 

2015 

3275 

3610 

4900 

5,295 

6,590 

7,020 

12 

535 

660 

1880 

2185 

3600 

4030 

5485 

6,000 

7,460 

8,035 

14 

540 

675 

1965 

2320 

3855 

4380 

5975 

6,620 

8,225 

8,950 

16 

545 

680 

2025 

2425 

4065 

4675 

6395 

7,165 

8,890 

9,775 

18 

545 

685 

2070 

2505 

4230 

4920 

6750 

7,630 

9,480 

10,520 

20 

545 

690 

2100 

2565 

4365 

5130 

7050 

8,060 

9,995 

11,190 

22 

545 

690 

2125 

2610 

4470 

5305 

7305 

8,425 

10,445 

11,795 

24 

545 

690 

2140 

2645 

4560 

5445 

7525 

8,750 

10,840 

12,340 

26 

545 

690 

2150 

2675 

4630 

5575 

7705 

9,035 

11,185 

12,830 

28 

545 

690 

2160 

2695 

4685 

5680 

7860 

9,280 

11,490 

13,270 

30 

545 

690 

2165 

2715 

4725 

5765 

7990 

9,500 

11,755 

13,670 

Very  great 

545 

690 

2180 

2770 

4910 

6230 

8725 

11,075 

13,635 

17,305 

92.  Cement  and  Concrete  Pipe. — Although  there  is  no  general 
recognition  of  a  difference  between  cement  and  concrete  pipe, 
there  is  a  tendency  to  term  manufactured  pipe  of  small  diameter 
cement  pipe,  and  large  pipes  or  pipes  constructed  in  place,  con- 
crete pipe.  Cement,  unlike  clay,  is  used  in  the  manufacture  of 


172  MATERIALS  FOR  SEWERS 

pipe  in  the  field  or  by  more  or  less  unskilled  operators  in  "  one 
man  "  plants.  Great  care  should  be  used  in  the  selection  of 
cement,  aggregate,  and  reinforcement  for  precast  cement  pipe 
since  the  shocks  to  which  it  is  subjected  in  transit  are  more  liable 
to  rupture  it  than  the  heavier  but  steadier  loads  imposed  on  it  in 
the  trench. 

The  United  States  Government,  various  scientific  and  engi- 
neering societies,  and  other  interested  organizations  have  col- 
laborated in  the  preparation  of  specifications  for  cement  and 
cement  tests.  These  specifications  can  be  found  in  Trans.  Am. 
Soc.  Civil  Engineers,  Vol.  82,  1918,  p.  166,  and  in  other  publica- 
tions. 

The  following  abstracts  have  been  taken  from  the  proposed 
tentative  specifications  for  Concrete  Aggregates,  of  the  Am. 
Society  for  Testing  Materials,  issued  June  21,  1921: 

1.  Fine  aggregate  shall  consist  of  sand,  stone  screen- 
ings, or  other  inert  materials  with  similar  characteristics, 
or   a    combination   thereof,    having    clean,    hard,    strong, 
durable  uncoated  grains,  free  from  injurious  amounts  of 
dust,  lumps,  soft  or  flaky  particles,  shale,  alkali,  organic 
matter,  loam  or  other  deleterious  substances. 

2.  Fine  aggregates    shall    preferably  be    graded  from 
fine  to  coarse,  with  the  coarser  particles  predominating, 
within  the  following  limits: 

Passing  No.  4  sieve 100  per  cent 

Passing  No.  50  sieve,  not  more  than 50  per  cent 

Weight  removed  by  elutrition  test,  not  more 

than 3  per  cent 

Sieves  shall  conform  to  the  U.  S.  Bureau  of  Standards 
specifications  for  sieves. 

3.  The  fine  aggregate  shall    be  tested  in  combination 
with  the  coarse  aggregate  and  the  cement  with  which  it 
is  to  be  used  and  in  the  proportions,  including  water,  in 
which  they  are  to  be  used  on  the  work,  in  accordance  with 
the  requirements  specified  in  Section  6.     ... 

7.  Coarse  aggregate  shall  consist  of  crushed  stone, 
gravel  or  other  approved  inert  materials  with  similar 
characteristics,  or  a  combination  thereof,  having  clean, 
hard,  strong,  durable,  uncoated  pieces  free  from  injurious 
amounts  of  soft,  friable,  thin,  elongated  or  laminated  pieces, 
alkali,  organic  or  other  deleterious  matter. 

The  following  Table  indicates  desirable  gradings,  in  per- 
centages, for  coarse  aggregate  for  certain  maximum  sizes. 


CEMENT  AND   CONCRETE   PIPE 
GRADINGS  OF  COARSE  AGGREGATES 


173 


Maximum 
Size  of 
Aggregate 
Inches 

Circular  Openings,  Inches 

Passing  Screen  Hav- 
ing Circular  Open- 
ings j  Inch  in  diam- 
eter, not  more  than 

3 

2| 

2 

1* 

U 

1 

i 

•i 

1 

3 
2* 
2 
11 
1* 
1 

3 
4 

100 

100 

100 

40-75 

15  per  cent 
15  per  cent 
15  per  cent 
15  per  cent 
15  per  cent 
15  per  cent 
15  per  cent 

40-75 

40-75 
100 

100 

100 

40-75 
100 

35-70 
40-75 

The  manufacture  of  small  size  cement  pipe  requires  relatively 
more  skill  than  equipment.  As  a  result  great  care  must  be 
observed  in  the  inspection  of  cement  pipe  and  in  the  enforcement 
of  specifications.  For  large  size  concrete  pipe  and  reinforced 
concrete  pipe  the  difficulty  of  holding  the  pipe  together  during 
transportation  and  lowering  into  the  trench  aid  in  insuring  a  good 
product. 

Cement  pipe  is  made  by  ramming  a  mixture  of  cement,  sand, 
and  water  into  a  cylindrical  mold  and  allowing  it  to  stand  until  set. 
The  mold  is  then  removed  and  the  pipe  stands  for  a  further  period 
of  time  to  become  cured.  The  selection  and  proportion  of 
materials,  the  amount  of  water,  the  method  of  ramming,  the 
period  of  setting,  the  length  of  time  of  curing,  and  the  control  of 
moisture  and  temperature  during  this  period  are  of  great  impor- 
tance in  the  resulting  product.  E.  S.  Hanson 1  states  that  the 
most  conservative  engineers  recommend  a  mixture  of  one  sack  of 
cement  to  2^  cubic  feet  of  aggregate  measured  as  loosely  thrown 
into  the  measuring  box.  In  making  up  the  aggregate,  clean  gravel 
or  broken  stone  up  to  J  inch  in  size  is  used.  The  American  Con- 
crete Institute  recommends  that  100  per  cent  pass  a  J-inch  screen, 
70  per  cent  a  J-inch  screen,  50  per  cent  a  No.  10,  40  per  cent  a 
No.  20,  30  per  cent  a  No.  30,  and  20  per  cent  a  No.  40.  The 
materials  should  be  carefully  graded  by  experiment  and  not 
guessed  at,  as  the  behavior  of  all  aggregates  is  not  the  same. 
Too  .coarse  an  aggregate  is  difficult  to  handle  in  manufacturing. 
-  l  Proceedings  Illinois  Society  of  Engineers,  1916,  page  81. 


174 


MATERIALS  FOR  SEWERS 


It  causes  loss  of  pipe  when  the  jacket  or  mold  is  removed  and 
results  in  rough  pipe,  stone  pockets,  and  pin  holes  through  which 
water  spurts  when  pressure  tests  are  applied.  Too  fine  an  aggre- 
gate causes  loss  of  strength  and  with  ordinary  mixtures  tends  to 
produce  a  pipe  which  will  show  seepage  under  internal  pressure 
tests.  The  amount  of  water  in  the  mixture  will  vary  from  15  to 
20  per  cent.  The  mixture  should  appear  dry  but  should  ball  in 
the  hand  under  some  pressure. 

The  mixture  can  be  rammed  into  the  molds  by  hand  or  machine. 
A  machine-made  pipe  is  preferable  as  it  produces  a  more  even  and 
stronger  product.  There  are  two  types  of  machines  for  this 
purpose.  One  type  consists  of  a  number  of  tamping  feet  which 
deliver  about  200  blows  to  the  minute  with  a  pressure  of  about 
800  pounds  per  square  inch  of  area  exposed.  In  the  other  type  a 
revolving  core  is  drawn  through  the  pipe,  packing  and  polishing 
the  concrete  as  it  is  pulled  through,  with  special  provision  for 

packing  the  bell  of  the  pipe. 
The  tamping  machines  can 
make  1,500  feet  of  small  size 
pipe  to  300  feet  of  24-inch 
pipe  in  a  day.  Machines  of 
the  second  type  can  make  750 
feet  of  8-inch  to  200  feet  of 
30-inch  pipe  in  30-inch  lengths 
in  9  hours.  The  inside  and 
outside  forms  for  a  24-inch 
pipe  are  shown  in  Fig.  74  as 
used  with  the  tamping  ma- 
chines. The  forms  are  swab- 
bed with  oil  before  being  filled 
in  order  to  facilitate  their 
removal.  In  making  a  Y- 
branch  or  other  special,  a 
hole  is  cut  in  the  pipe  or 

Bottom  vie*.  Bottom  view.          mold   the   size    of   the    joining 

FIG.  74.— Details  of  24-Inch  Concrete  pipe  which  is  then  set  in  place 
Pipe  Form.  and   the    joint    wiped    smooth 

with  cement. 

After  the  removal  of  the  mold  the  pipe  may  be  cured  by  the 
water  or  the  steam  process.  Hanson  states: 


Inside 
Form 


u 


Outside 
Form 


IK  5 


Section. 


Elevation. 


CEMENT  AND   CONCRETE   PIPE 


175 


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eococococoeococo 


«  ^  w  co  co  co 


176  MATERIALS  FOR  SEWERS 

By  the  former  the  pipe  are  simply  set  on  the  floor  of 
the  plant  and  as  soon  as  they  are  sufficiently  strong  so 
that  they  can  be  sprinkled  with  water  without  falling 
down;  sprinkling  is  commenced  and  continued  at  such 
intervals  for  6  or  7  days  that  the  pipe  will  be  moist  at  all 
times.  This  is  a  slower  process  than  steam  curing.  It  is 
also  less  uniform  and  less  subject  to  control  than  where 
the  product  is  cured  by  steam. 

In  the  steam  process  the  pipe  is  exposed  to  low-pressure  steam 
with  plenty  of  moisture  in  a  closed  receptacle  for  24  hours,  or 
until  hardened.  It  has  been  found  by  tests  that  pipes  sprinkled 
for  28  days  are  as  strong  as  steam-cured  pipes. 

The  dimensions  of  cement  concrete  sewer  pipe  as  recommended 
by  the  Am.  Society  for  Testing  Materials  are  shown  in  Table  38. 

The  following  has  been  abstracted  from  the  description  of  the 
manufacture  of  one  form  of  concrete  pipe  by  G.  C.  Bartram.1 
All  pipe  are  manufactured  in  4-foot  lengths  near  the  site  at  which 
they  are  to  be  installed  because  of  their  great  weight,  for  example, 
36-inch  pipe  weighs  one  ton.  The  plant  for  the  manufacture  of 
the  pipe  consists  of  cast-iron  bottom  and  top  rings  for  each  size 
to  be  used  on  the  job,  and  inside  and  outside  steel  casings. 
There  are  three  bases  for  each  steel  casing  as  the  pipes  stand  on 
the  bases  for  72  hours  and  the  steel  casing  remains  on  for  only 
24  hours  after  the  concrete  has  been  poured.  The  pipes  are  then 
lifted  off  the  bases  and  stored  for  aging.  The  pipes  are  cast  with 
the  spigot  end  up. 

The  concrete  is  ordinarily  mixed  in  the  proportions  of  1  :  2  :  4. 
The  materials  are  placed  in  the  mixer  in  the  following  order: 
first,  the  stone,  then  the  sand,  then  the  cement,  and  finally  the 
water.  Sufficient  water  is  added  to  make  the  concrete  flow  freely. 
In  cold  weather  or  for  a  hurry-up  job  the  molds  are  covered  with 
canvas  and  are  steamed  for  2  or  3  hours  immediately  after  the 
concrete  is  poured.  The  molds  are  then  removed  but  the  pipe 
should  be  steamed  before  use.  Otherwise  they  are  allowed  to 
stand  72  hours,  as  explained  above.  In  cold  weather  the  steam  is 
used  to  prevent  freezing  and  not  to  hasten  the  completion  of  the 
pipe. 

One  layer  or  ring  of  reinforcement  is  used  for  sizes  from  24  to 
48  inches  and  two  layers  or  rings  for  larger  pipe.  A  type  of  rein- 

1  Municipal  Engineers'  Journal  for  April,  1918. 


CEMENT  AND  CONCRETE   PIPE 


177 


forcement  sometimes  used  is  the  American  Steel  and  Wire  Com- 
pany's Triangular  Mesh,  an  illustration  of  which  is  shown  in 
Fig.  75.  The  wire  mesh  is  cut  to  fit  and  is  placed  in  a  slot  in  the 
cast-iron  base.  The  slot  is  then  filled  with  sand  so  that  the  con- 


^ 


Style*  ZZ 


8-0"- 
Section  A-A. 


Section  B-B 

FIG.  75. — Triangle  Mesh  Reinforced  Concrete  Pipe. 

As  made  by  the  Am.  Concrete  Pipe  and  Pile  Co.,  Chicago. 


Tongue  and  Groove  Lock  Joint, 

Joint  Patented 

Hub  and  Spigot  Joint. 

A    Three  Types  of  Joints  Commonly  Used  in  Reinforced  Concrete  Pipe 
Sewer  Construction 


Single  Line  Double  Line  Elliptical 

Reinforcing  Reinforcing  Reinforcing. 

B     Three    Methods    of    Reinforcing    Concrete  Pipe 

FIG.  76. — Methods  of  Joining  and  Reinforcing  Concrete  Pipe. 

Crete  cannot  enter,  thus  leaving  a  portion  of  the  reinforcement 
exposed.  The  inside  reinforcement  extends  through  and  out  of 
the  spigot  of  the  completed  pipe.  In  the  trench  the  two  rein- 
forcements overlap  in  the  key-shaped  space  left  on  the  inside  of 


178 


MATERIALS  FOR  SEWERS 


the  pipe  by  the  design  of  the  bell  and  spigot.  This  space  is  shown 
in  Fig.  76  A.  When  the  pipe  is  placed  in  the  trench  the  key-shaped 
space  is  plastered  with  mortar  and  a  piece  is  knocked  out  of  the 
bell  to  receive  the  grout  with  which  the  joint  is  closed.  A  spring 
steel  band  is  then  put  on  the  outside  of  the  joint  and  grout  poured 
into  the  hole  at  the  top.  The  band  is  removed  as  soon  as  the 
joint  materials  have  set. 

The  rules  for  the  reinforcement  of  concrete  pipe  recommended 
in  Volume  XV,  1919,  of  the  Transactions  of  the  Concrete  Insti- 
tute are  as  follows: 

No  reinforcement  is  approved  for  pipe  between  30  and 
60  inches  in  diameter  or  in  rock  or  hard  soils.  For  pipe 
36  inches  in  diameter  or  less  the  minimum  thickness  of 
shell  shall  be  5  inches.  For  60-inch  pipe  the  minimum 
thickness  shall  be  7  inches  with  intermediate  sizes  in  pro- 
portion. Reinforcement  for  circular  pipe  shall  consist  of 
one  or  two  rings  of  circular  wire  fabric  or  rods  of  the  areas 
shown  in  Table  39.  All  sewers  near  the  surface  and  sub- 
ject to  vibration  should  be  reinforced.  For  sewers  6  feet 
or  less  in  diameter  the  reinforcement  should  consist  of 
at  least  \  of  1  per  cent  of  the  area  of  the  concrete.  It 
should  be  placed  near  the  inside  at  the  crown  and  near 

TABLE  39 

REINFORCEMENT  FOR  CIRCULAR  CONCRETE  SEWER  PIPE 
(See  Vol.  XV,  Proceedings  Am.  Concrete  Institute) 


Mini- 

Cross 

Mini- 

• 

Cross 

Diam- 
eter 

mum 
Thick- 

Number 

Sec- 
tional 

Diam- 
eter 

mum 
Thick- 

Number 

Sec- 
tional 

in 
Inches 

ness 
of  Shell 

of  Rings 

Area  of 
Each 

in 
Inches 

ness 
of  Shell 

of  Rings 

Area  of 
Each 

in 
Inches 

Ring 

in 
Inches 

Ring 

24 

3 

1 

.058 

48 

5 

2 

.107 

27 

3 

1 

.068 

54 

5| 

2 

.125 

30 

3| 

1 

.080 

60 

6 

2 

.146 

33 

4 

1 

.107 

66 

6| 

2 

.168 

36 

4 

1 

.146 

72 

7 

2 

.180 

39 

4 

1 

.146 

84 

8 

2 

.208 

42 

41 

1 

.153 

96 

9 

2 

.245 

PROPORTIONING  OF  CONCRETE  179 

the  outside  at  the  haunches.  If  large  horizontal  pressures 
are  expected  the  pipe  should  be  reinforced  for  these  reverse 
stresses,  which  involves  placing  the  reinforcement  near 
the  outside  at  the  crown  and  near  the  inside  at  the 
haunches.  The  minimum  thickness  of  the  walls  of  sewers 
greater  than  6  feet  in  diameter  with  flat  bottom  and  arch, 
with  or  without  side  walls,  should  be  8  inches. 

Three  methods  for  the  reinforcement  of  concrete  sewers  are  shown 
in  Fig.  76  B. 

93.  Proportioning  of  Concrete. — In  the  proportioning  of  con- 
crete questions  of  strength,  of  permeability,  and  of  workability  * 
may  need  consideration.  All  of  these  qualities  are  affected  by 
the  amount  of  cement,  the  nature  and  gradation  and  relative 
proportions  of  the  fine  and  the  coarse  aggregate,  and  the  amount 
of  mixing  water  used. 

Other  things  being  equal  the  strength  varies  with  the  amount 
of  cement  put  into  the  concrete.  For  the  same  amount  of  cement 
and  the  same  consistency  of  the  mixture,  the  strength  increases 
with  increased  density  of  concrete  (that  is,  with  decreased  voids), 
and  the  effort  should  be  made  so  to  proportion  the  fine  and  coarse 
aggregates  as  to  produce  the  densest  concrete  (least  voids)  with 
the  aggregates  available.  For  the  same  consistency,  the  strength 
then  will  vary  with  the  ratio  of  the  amount  of  cement  to  the 
amount  of  the  voids. 

So  far  as  the  mixing  water  is  concerned,  the  greatest  strength 
in  the  concrete  will  be  attained  at  a  rather  dry  mix;  that  which 
produces  the  least  volume  of  concrete.  The  addition  of  more 
water  results  in  a  concrete  of  less  strength;  40  per  cent  more 
water  may  give  a  concrete  of  less  than  half  the  normal  strength. 
The  reduction  in  strength  is  then  very  marked  for  the  wetter 
mixes,  and  the  water  content  used  is  a  feature  of  considerable 
importance  in  the  design  of  concrete  mixtures. 

Permeability  is  affected  by  the  same  elements  as  strength, 
but  the  size  and  discontinuity  of  the  pores  have  a  greater  influence. 

Workability  is  an  important  quality;  in  some  respects  it  will 
have  to  be  obtained  at  the  expense  of  strength.  Increasing  the 
amount  of  mixing  water  increases  the  workability  of  the  mixtures, 
with  a  resulting  decrease  in  strength  which  may  have  to  be 
accepted  or  else  overcome  by  increasing  the  cement  in  the  mix. 
1  Workability  involves  ease  in  placing  and  smoothness  of  working. 


180  MATERIALS  FOR  SEWERS 

An  excess  of  water  is  often  used  unnecessarily  through  ignorance 
of  the  injurious  results.  A  high  proportion  of  coarse  aggregate, 
up  to  a  certain  limit,  will  give  concrete  of  high  strength,  but  the 
mixture  will  be  harsh-working  and  not  easy  to  place.  Lower 
proportions  of  coarse  aggregate  will  give  greater  workability  and 
better  uniformity  of  product,  the  latter  being  an  important 
matter.  It  is  apparent  that  the  degree  of  workability  of  the  mix- 
ture needed  will  depend  upon  the  nature  of  the  construction — for 
a  pavement  where  the  concrete  will  receive  substantial  tamping 
or  working  the  water  content  may  be  much  less  than  that  which 
may  need  to  be  used  in  placing  concrete  around  reinforcement  in 
narrow  members,  or  where  little  tamping  or  spading  can  be  done. 
The  nature  of  the  work  will  affect  the  standard  of  consistency  to  be 
specified. 

The  proportioning  of  the  concrete  should  then  be  dependent 
upon  the  needs  of  the  structure  and  the  manner  of  placing  the 
concrete.  The  proportions  selected  should  be  carefully  adhered 
to  and  especially  should  care  be  taken  to  see  that  the  right  quan- 
tity of  mixing  water  is  used. 

The  materials  are  commonly  measured  volumetrically  (by 
bulk).  Because  of  the  variations  which  are  introduced  by  volu- 
metric measurement  of  the  materials  by  the  presence  of  varying 
degrees  of  moisture,  measurements  by  weight  would  be  more 
accurate,  but  these  would  also  be  affected  by  differences  in  the 
specific  gravity  of  the  materials.  The  methods  of  measuring, 
the  allowance  for  moisture,  as  well  as  the  proportions  of  the 
materials,  should  be  specified. 

The  methods  for  proportioning  concrete  are: 

(1)  Arbitrarily  selected  proportions. 

(2)  Proportions  based  on  minimum  voids. 

(3)  Proportions  based  on  trial  mixtures. 

(4)  Proportions  based  on  a  sieve  analysis  curve. 

(5)  Proportions  based  on  the  surface  area  of  the  aggre- 


(6)  Proportions  based  on  the  water-cement  ratio  and 
the  fineness  modulus. 

(7)  Proportions   based   on  mortar-voids   and   cement- 
voids  ratio. 

Arbitrarily   selected   proportions   are   in   quite   general   use; 
they  are  intended  to  apply  to  the  materials  most  commonly  used 


PROPORTIONING   OF  CONCRETE  181 

in  the  vicinity  of  the  work.  The  most  common  practice  is  to 
use  twice  as  great  a  volume  of  coarse  aggregate  as  fine  aggregate, 
as  for  instance  1  part  cement,  2  parts  fine  aggregate,  and  4  parts 
coarse  aggregate.  Decreasing  the  ratio  of  coarse  aggregate  to 
fine  aggregate  may  give  a  more  easily  worked  mix  or  require 
relatively  less  water  for  a  given  workability,  and  in  some  cases  it 
will  be  proper  to  increase  tnis  ratio  and  thus  secure  an  increase  of 
strength.  Judgment  and  experience  with  given  materials  may 
warrant  changes  from  a  stated  ratio.  The  proportions  are  now 
frequently  given  as  one  part  cement  to  a  certain  number  of  parts 
of  the  mixed  aggregate,  leaving  the  proportions  of  the  fine  to 
coarse  to  be  determined  otherwise,  since  small  variations  in  the 
relation  of  these  will  not  greatly  affect  the  strength.  Proportions 
in  common  use  are:1 

Mortar  for 


Laying  brick  and  stone  masonry from  1:0    to  1 

Filling  joints  in  sewer  pipe 1:0    to  1 

Surfaces,  floors,  sidewalks,  pavements. .  1  :  0    to  1 

Waterproof  linings 1:0    to  1 

Cement,  bricks,  and  blocks 1  :  2J-  to  1 


Concrete  for 

Gravity  retaining  walls,  heavy  founda- 
tions, structures  needing  mass  more 
than  strength from  1:3:6  to  1  :  4  :  8 

Retaining  walls,  piers,  sewers,  pave- 
ments, foundations,  and  work  requir- 
ing strength.  (Compressive  strength 
in  28  days,  1,500  to  2,000  pounds  per 
square  inch) from  1  :  2  :  *4  to  1  :  3  :  6 

Floors,  beams,  pavements,  reinforced 
concrete,  arch  bridges,  low-pressure 
tanks.  (Compressive  strength  in  28 
days,  2,000  to  3,000  pounds  per  square 
inch) from  1  :  1£  :  3  to  1  :  2\  :  4£ 

Reinforced  concrete  columns,  conduit 
pipe,  impervious  concrete.  (Com- 
pressive strength  in  28  days,  3,000  to 
4,000  pounds  per  square  inch) .  .  .  from  1:1:2  to  1  :  1-J-  :  3 

The  usual  method  of  proportioning  based  on  minimum  voids 

is  to  assume  that  the  particles  of  fine  aggregate  should  fill  the 

voids  in  the  coarse  aggregate  and  that  the  particles  of  the  cement 

will  fill  the  voids  in  the  fine  aggregate.     About  5  to  10  per  cent 

1  Johnson's  Materials  of  Construction,  5th  Edition,  1918,  p.  432. 


182 


MATERIALS  FOR  SEWERS 


additional  fine  aggregate  is  generally  added  to  push  the  particles 
of  the  coarse  aggregate  apart  and  thus  give  a  more  easily  worked 
concrete  and  one  freer  from  void  spaces.  This  method  is  inaccu- 
rate, principally  because  of  the  effect  of  the  moisture  on  the  volume 
of  the  voids,  and  because  the  effect  on  the  volume  by  the  addi- 
tion of  water  is  unknown. 

Trial  mixtures  may  be  made  by  carefully  weighing  each  of  the 
ingredients  and  then  combining  them  to  give  a  workable  concrete.. 
Using  a  given  amount  of  cement,  the  proportion  of  ingredients,  of 
the  same  total  weight,  which  will  give  the  least  volume  and  there- 
fore the  densest  concrete  is  adopted.  When  making  the  compari- 
son the  consistency  of  the  mixes  must  be  maintained  constant. 

Proportioning  may  be  based  on  an  ideal  sieve  analysis  curve 
of  the  mixed  cement  and  aggregates.  The  sieve  analysis  of  the 
aggregates  is  made  by  screening  a  predetermine  d  weight  of  the 
sample  through  a  series  of  5  to  8  sieves  graded  in  size  from  slightly 
below  the  size  of  the  largest  particle  to  slightly  above  the  smallest 
particle  of  the  aggregate.  The  analysis  is  then  expressed  in  the 
form  of  a  curve.  The  ideal  curve,  according  to  Fuller,1  is  shown 
in  Fig.  77. 

100 


0.25 


0.50  0.75  1.00 

Diameter  of  Particle  in    Inches. 


1.25 


1.50 


FIG.  77. — Gravel  Analysis. 

The  dotted  line  indicates  the  ideal  combination  of  the  coarse  and  fine  portions.     The  heavy 
full  line  indicates  the  combination  attained. 

1  Trans.  Am.  Society  of  Civil  Engineers,  Vol.  59,    1907,  p.  146. 


WATERPROOFING  CONCRETE  183 

The  method  of  proportioning  concrete  by  surface  areas  is 
based  on  the  theory  that  the  strength  of  a  concrete  depends  on  the 
amount  of  cement  used  in  proportion  to  the  surface  area  01  the 
aggregates.1 

The  proportioning  of  concrete  on  the  basis  of  a  water-cement 
ratio  and  a  fineness  modulus  was  introduced  by  Prof.  D.  A. 
Abrams.2  It  is  based  on  the  theory  that  with  fixed  conditions 
of  aggregate,  moisture,  etc.,  the  ratio  of  water  to  cement  deter- 
mines the  strength  of  the  concrete. 

A  method  of  proportioning  concrete  by  determining  experi- 
mentally the  voids  in  mortars  made  up  with  a  given  amount  of 
sand  and  definite  proportions  of  cement,  and  then  calculating 
the  voids  in  the  concrete  made  up  by  adding  a  definite  amount 
of  coarse  aggregate  to  the  mixture,  has  been  developed.3  Ibc 
method  is  based  on  the  theory  that  the  strength  of  the  concrete 
is  a  known  function  of  the  ratio  of  the  volume  of  cement  to  the 
volume  of  the  voids  in  the  concrete.  The  effect  of  varying  the 
proportion  of  the  ingredients,  including  an  increase  in  the  amount 
of  mixing  water  beyond  that  required  to  give  the  densest  mixture, 
may  be  found  by  the  method,  and  a  comparison  may  be  made  of 
results  obtainable  with  different  classes  of  fine  and  coarse  aggre- 
gates. 

Arbitrarily  selected  proportions,  proportions  based  on  voids, 
and  proportions  based  on  trial  mixtures  are  usually  satisfactory 
for  small  jobs  where  the  amount  of  materials  involved  is  not  large. 
Where  the  saving  in  materials  will  permit,  more  accurate  methods 
should  be  used.  The  methods  can  be  studied  more  fully  by 
reference  to  the  original  articles  quoted  in  the  footnotes,  of  to  the 
following  texts: 

Materials  of  Construction,  Johnson,  5th  Edition,  1918. 
Materials   of   Engineering,  H.  F.  Moore,    2d  Edition,  1920. 
Masonry  Construction,  I.  O.  Baker,  10th  Edition,  1912. 
Concrete   Engineer's   Handbook,    Hool   and   Johnson,  1918. 
Concrete,  Plain  and  Reinforced,  Taylor  and  Thompson,  1916. 

1  L.  N.  Edwards,  Trans.  Am.  Society  Testing  Materials,  1918,  and  R.  B. 
Young,  Eng.  News-Record,  Vol.  82,  1919,  p.  33. 

2  Bulletin  No.  1,  Structural  Materials  Research  Laboratory,  Lewis  Insti- 
tute, Chicago,  Illinois. 

3  Proportioning  Concrete  by  Voids  in  the  Mortar,  A.  N.  Talbot,  read 
before  -Am.  Society  Testing  Materials,  June  22,   1921.     Abstract  in  Eng. 
News-Record,  Vol.  87,  1921,  p.  147. 


184  MATERIALS  FOR  SEWERS 

94.  Waterproofing  Concrete. — The  waterproofing  of  concrete 
is  most  satisfactorily  done  by  making  dense  mixtures.     In  practice 
such  substances  as  hydrated  lime,  clay,  alum  and  soap,  and  pro- 
prietary compounds  such  as  Ceresit,  Medusa,  etc.,  are  frequently 
mixed  with  the  concrete  under  the  theory  that  these  very  fine 
substances  will  fill  any  remaining  voids  and  render  the  concrete 
impervious.     The  specifications  of  the  Joint  Committee  issued  on 
June  4,  1921,  are  much  briefer  and  contain  less  detailed  instruc- 
tion than  those  issued  earlier.1      The  earlier  instructions  follow. 

Many  expedients  have  been  resorted  to  for  making 
concrete  impervious  to  water.  Experience  shows,  however, 
that  when  mortar  or  concrete  is  proportioned  to  obtain 
the  greatest  practicable  density  and  is  mixed  to  the  proper 
consistency,  the  resulting  mortar  or  concrete  is  impervious 
under  moderate  pressure. 

On  the  other  hand  concrete  of  dry  consistency  is  more 
or  less  pervious  to  water,  and,  though  compounds  of  vari- 
ous kinds  have  been  mixed  with  the  concrete  or  applied 
as  a  wash  to  the  surface,  in  an  effort  to  offset  this  defect, 
these  expedients  have  generally  been  disappointing,  for 
the  reason  that  many  of  these  compounds  have  at  best 
but  temporary  value,  and  in  time  lose  their  power  of  impart- 
ing impermeability  to  the  concrete. 

In  the  case  of  subways,  long  retaining  walls,  and  reser- 
voirs, provided  the  concrete  itself  is  impervious,  cracks 
may  be  so  reduced,  by  horizontal  and  vertical  reinforcement 
properly  proportioned  and  located,  that  they  will  be  too 
minute  to  permit  leakage,  or  will  be  closed  by  infiltration 
of  silt. 

Asphaltic  or  coal  tar  preparations  applied  either  as  a 
mastic  or  as  a  coating  on  felt  cloth  or  fabric,  are  used  for 
waterproofing,  and  should  be  proof  against  injury  by  liquids 
or  gases. 

For  retaining  and  similar  walls  in  direct  contact  with 
the  earth,  the  application  of  one  or  two  coatings  of  hot 
coal  tar  pitch,  following  a  painting  with  a  thin  wash  of 
coal  tar  dissolved  in  benzol,  to  the  thoroughly  dried  sur- 
face of  concrete  is  an  efficient  method  of  preventing  the 
penetration  of  moisture  from  the  earth. 

Tar  paper  and  asphaltic  compounds  are   not   often   used  in 
sewer  work  as  absolute  imperviousness  is  seldom  necessary. 

95.  Mixing  and  Placing  Concrete. — Careful  workmanship  is 
desirable  in  the  mixing  and  placing  of   concrete  in  sewers    since 

1  Trans.  Am.  Society  of  Civil  Engineers,  Vol.  81,  1917,  p.  1122. 


MIXING  AND  PLACING  CONCRETE  185 

water-tight  construction  is  desired.  Because  of  the  difficulty  of 
inspecting  concrete  in  wet,  dark  and  crowded  excavations,  and 
the  careless  habits  of  workmen  experienced  in  concrete  sewer 
construction,  the  highest  class  of  concrete  work  cannot  be  expected. 
The  situation  is  met  by  designing  thick  walls  as  shown  in  the 
sections  illustrated  in  Fig.  22  and  23. 

In  the  report  of  the  Joint  Committee  on  Concrete  and  Rein- 
forced Concrete  in  Transactions  of  the  American  Society  of 
Civil  Engineers  for  1917,  on  page  1101  the  recommendation 
is  made  concerning  the  mixing  and  placing  of  concrete  as 
follows : l 

The  mixing  of  concrete  should  be  thorough  and  should 
continue  until  the  mass  is  uniform  in  color  and  is  homo- 
geneous. As  the  maximum  density  and  greatest  strength 
of  a  given  mixture  depends  largely  on  thorough  and  com- 
plete mixing,  it  is  essential  that  this  part  of  the  work 
should  receive  special  attention  and  care. 

Inasmuch  as  it  is  difficult  to  determine  by  visual 
inspection  whether  the  concrete  is  uniformly  mixed,  especi- 
ally where  aggregates  having  the  color  of  cement  are  used, 
it  is  essential  that  the  mixing  should  occupy  a  definite 
period  of  time.  The  minimum  time  will  depend  on  whether 
the  mixing  is  done  by  machine  or  hand. 

(a)  Measuring  Ingredients:  Methods  of  measurement 
of  the  various  ingredients  should  be  used  which  will  secure 
at  all  times  separate  and  uniform  measurements  of  cement, 
fine  aggregate,  coarse  aggregate  and  water. 

(6)  Machine  Mixing:  The  mixing  should  be  done  in 
a  batch  machine  mixer  of  a  type  which  will  insure  the 
uniform  distribution  of  the  materials  throughout  the  mass, 
and  should  continue  for  the  minimum  time  of  1J  minutes 
after  all  the  ingredients  are  assembled  in  the  mixer.  For 
mixers  of  2  or  more  cubic  yards  capacity,  the  minimum 
time  of  mixing  should  be  2  minutes.  Since  the  strength 
of  the  concrete  is  dependent  on  thorough  mixing,  a  longer 
time  than  this  minimum  is  preferable.  It  is  desirable 
to  have  the  mixer  equipped  with  an  attachment  for  auto- 
matically locking  the  discharging  device  so  as  to  prevent 
the  emptying  of  the  mixer  until  all  the  materials  have  been 
mixed  together  for  the  minimum  time  required  after  they 
are  assembled  in  the  mixer.  Means  should  be  provided 

1  See  also  Tentative  Specifications  for  Concrete  and  Reinforced  Concrete 
submitted  by  the  Joint  Committee  to  its  Constituent  Organizations,  June 
4,  1921: 


186  MATERIALS  FOR  SEWERS 

to  prevent  aggregates  being  added  after  the  mixing  has 
commenced.  The  mixer  should  also  be  equipped  with 
water  storage,  and  an  automatic  measuring  device  which 
can  "be  locked  if  desired.  It  is  also  desirable  to  equip 
the  mixer  with  a  device  recording  the  revolutions  of  the 
drum.  The  number  of  revolutions  should  be  so  regulated 
as  to  give  at  the  periphery  of  the  drum  a  uniform  speed. 
About  200  feet  per  minute  seems  to  be  the  best  speed  in 
the  present  state  of  the  art. 

(c)  Hand  Mixing:    Hand  mixing  should  be  done  on 
a    watertight    platform    and    especial    precautions    taken 
after  the  water  has  been  added,  to  turn  all  the  ingredients 
together  at  least  6  times,  and  until  the  mass  is  homo- 
geneous in  appearance  and  color. 

(d)  Consistency:    The  materials  should  be  mixed  wet 
enough  to  produce  a  concrete  of  such  a  consistency  as  will 
flow  sluggishly  into  the  forms  and  about  the  metal  reinforce- 
ment when  used,  and  which  at  the  same  time  can  be  con- 
veyed from  the  mixer  to  the  forms  without  separation 
of  the  coarse  aggregate  from  the  mortar.     The  quantity 
of  water  is  of  the  greatest  importance  in  securing  concrete 
of  maximum  strength  and  density;   too  much  water  is  as 
objectionable  as  too  little. 

(e)  Retempering :  The  remixing  of  concrete  and  mortar 
that  has  partly  reset  should  not  be  permitted. 

Placing  Concrete 

(a)  Methods:  Concrete  after  the  completion  of  the 
mixing  should  be  conveyed  rapidly  to  the  place  of  final 
deposit;  under  no  circumstances  should  concrete  be  used 
that  has  partly  set. 

Concrete  should  be  deposited  in  such  a  manner  as  will 
permit  the  most  thorough  compacting  such  as  can  be 
obtained  by  working  with  a  straight  shovel  or  slicing  tool 
kept  moving  up  and  down  until  all  the  ingredients  are  in 
their  proper  place.  Special  care  should  be  exercised  to 
prevent  the  formation  of  laitance;  where  laitance  has 
formed  it  should  be  removed,  since  it  lacks  strength  and 
prevents  a  proper  bond  in  the  concrete. 

Care  should  be  taken  that  the  forms  are  substantial 
and  thoroughly  wetted  (except  in  freezing  weather)  or 
oiled,  and  that  the  space  to  be  occupied  by  the  concrete 
is  free  from  all  debris.  When  the  placing  of  concrete 
is  suspended,  all  necessary  grooves  for  joining  future  work 
should  be  made  before  the  concrete  has  set. 

When  work  is  resumed  concrete  previously  placed 
should  be  roughened,  cleansed  of  foreign  material  and 


MIXING  AND  PLACING  CONCRETE  187 

laitance,  thoroughly  wetted  and  then  slushed  with  a  mortar 
consisting  of  one  part  Portland  cement  and  not  more  than 
2  parts  of  fine  aggregate. 

The  surfaces  of  concrete  exposed  to  premature  drying 
should  be  kept  covered  and  wet  for  at  least  7  days. 

Where  concrete  is  conveyed  by  spouting,  the  plant 
should  be  of  such  a  size  and  design  as  to  insure  a  practically 
continuous  stream  in  the  spout.  The  angle  of  the  spout 
with  the  horizontal  should  be  such  as  to  allow  the  concrete 
to  flow  without  separation  of  the  ingredients;  in  general 
an  angle  of  about  27  degrees  or  1  vertical  to  2  horizontal 
is  good  practice.  The  spout  should  be  thoroughly  flushed 
with  water  before  and  after  each  run.  The  delivery  from  the 
spout  should  be  as  close  as  possible  from  the  point  of  deposit. 
Where  the  discharge  must  be  intermittent,  a  hopper  should 
be  provided  at  the  bottom.  Spouting  through  a  vertical 
pipe  is  satisfactory  when  the  flow  is  continuous;  when  it 
is  checked  and  discontinuous  it  is  highly  objectionable  unless 
the  flow  is  checked  by  baffle  plates. 

(6)  Freezing  Weather:  Concrete  should  not  be  mixed 
or  deposited  at  a  freezing  temperature,  unless  special 
precautions  are  taken  to  prevent  the  use  of  materials 
covered  with  ice  cystals  or  containing  frost,  and  to  prevent 
the  concrete  from  freezing  before  it  has  set  and  sufficiently 
hardened. 

As  the  coarse  aggregate  forms  the  greater  portion  of 
the  concrete,  it  is  particularly  important  that  this  material 
be  warmed  to  well  above  the  freezing  point. 

The  enclosing  of  a  structure  and  the  warming  of  a  space 
inside  the  enclosure  is  recommended,  but  the  use  of  salt 
to  lower  the  freezing  point  is  not  recommended. 

(c)  Rubble   Concrete:    Where  the   concrete  is  to  be 
deposited  in  massive  work,  its  value  may  be  improved 
and  its  cost  materially  reduced  by  the  use  of  clean  stones 
saturated  with  water,  thoroughly  embedded  in  and  com- 
pletely surrounded  by  concrete. 

(d)  Under  Water:  In  placing  concrete  under  water,  it  is 
essential  to  maintain  still  water  at  the  place  of  deposit. 
With  careful  inspection  the  use  of  tremies,  properly  designed 
and  operated,  is  a  satisfactory  method  of  placing  concrete 
through  water.     The  concrete  should  be  mixed  very  wet 
(more  so  than  is  ordinarily  permissible)  so  that  it  will  flow 
readily  through  the  tremie  and  into  place  with  practically 
a  level  surface. 

The  coarse  aggregate  should  be  smaller  than  ordinarily 
used  and  never  more  than  one  inch  in  diameter.  The 
use  of  gravel  facilitates  the  mixing  and  assists  the  flow. 
The  mouth  of  the  tremie  should  be  buried  in  the  concrete 


188  MATERIALS  FOR  SEWERS 

so  that  it  is  at  all  times  entirely  sealed  and  the  surrounding 
water  prevented  from  forcing  itself  into  the  tremie.  The 
concrete  will  then  discharge  without  coming  in  contact 
with  the  water.  The  tremie  should  be  suspended  so  that 
it  can  be  lowered  quickly  when  it  is  necessary  either  to 
choke  off  or  to  prevent  too  rapid  flow.  The  lateral  flow 
preferably  should  not  be  over  15  feet. 

The  flow  should  be  continuous  in  order  to  produce  a 
monolithic  mass  and  to  prevent  the  formation  of  laitance 
in  the  interior. 

In  case  the  flow  is  interrupted  it  is  important  that  all 
laitance  be  removed  before  proceeding  with  the  work. 

In  large  structures  it  may  be  necessary  to  divide  the 
mass  of  concrete  into  several  small  compartments  or  units 
to  permit  the  continuous  filling  of  each  one.  With  proper 
care  it  is  possible  in  this  manner  to  obtain  as  good  results 
under  water  as  in  the  air. 

A  less  desirable  method  is  the  use  of  the  drop  bottom 
bucket.  Where  this  method  is  used  the  bottom  of  the 
bucket  should  be  released  when  in  contact  with  the  surface 
of  the  place  of  deposit. 

Concrete  sewers  should  be  constructed  in  longitudinal  sections 
in  a  continuous  operation  without  interruption  for  the  entire 
invert,  side  walls,  or  arch.  In  pouring  the  concrete  it  should  be 
kept  level  in  the  forms  and  should  rise  evenly  on  each  side  of  the 
sewer.  All  rough  places  in  the  concrete  should  be  finished  smooth 
by  brushing  with  a  grout  of  neat  cement  and  water  and  honey- 
combs should  be  filled  with  neat  cement  or  a  one-to-one  mortar. 

96.  Sewer  Brick. — The  quality  of  brick  used  in  sewers  is 
seldom  specified  with  the  minute  care  that  is  taken  in  the  speci- 
fications for  concrete,  iron,  and  certain  other  materials  of  con- 
struction, as  inferior  materials  in  brick  are  more  easily  detected. 
The  specifications  of  the  Baltimore  Sewerage  Commission  for 
sewer  brick  are : 

Sewer  brick  shall  be  whole,  new  bricks  of  the  best 
quality,  of  uniform  standard  size,  with  straight  and  parallel 
edges  and  square  corners ;  they  shall  be  of  compact  texture, 
burned  hard  and  entirely  through,  free  from  injurious 
cracks  and  flaws,  tough  and  strong,  and  shall  have  a  clear 
ring  when  struck  together.  The  sides,  ends  and  faces  of 
all  bricks  shall  be  plane  surfaces  at  right  angles  and  parallel 
to  each  other.  Bricks  of  any  one  make  shall  not  vary 
more  than  j^th  of  an  inch  in  thickness,  nor  more  than 


VITRIFIED  SEWER  BLOCK  189 

|th  of  an  inch  in  width  or  length,  from  the  average  of  the 
samples  submitted  for  approval. 

The  truest  bricks  shall  be  used  in  the  face  of  the  masonry 
and  the  exposed  surfaces  shall  be  true  and  smooth  planes. 

All  bricks  delivered  for  use  shall  be  culled  by  the  Con- 
tractor when  required.  No  brick  thrown  out  in  the  culling 
shall  be  used  in  any  work  done  under  any  contract  of  the 
Sewerage  Commission,  except  that  the  best  of  the  culls 
may  be  used  in  manholes,  above  the  level  of  the  top  of 
the  sewer,  if  permitted  by  the  Engineer. 

The  average  amount  of  water  absorbed  by  the  bricks, 
after  being  thoroughly  dried  and  then  immersed  for  24 
hours,  shall  not  exceed  6  per  cent.  All  bricks  shall  be 
uniform  in  quality  and  percentage  of  absorption. 

Whenever  vitrified  bricks  are  required  in  the  invert 
of  the  sewer,  they  shall  be  smooth,  hard,  tough,  and  of 
such  durability  as  will  fit  them  for  this  use.  They  shall 
be  of  standard  size,  well  and  uniformly  burned,  thoroughly 
vitrified  throughout,  and  free  from  warps,  cracks,  and 
other  defects.  The  surfaces  and  edges  shall  be  true  and 
straight  and  the  corners  sharp  and  square.  They  shall 
be  in  every  respect  satisfactory  to  the  Engineer,  and  in  all 
respects  equal  to  the  sample  in  the  office  of  the  Engineer. 

The  remaining  paragraphs  of  the  specifications  deal  with  the 
manner  in  which  samples  shall  be  submitted  and  the  necessity  for 
conformity  between  the  samples  submitted  and  the  bricks  used. 

A  common  size  of  brick  in  use  for  sewers  is  2JX4X8J  inches, 
but  the  variations  in  size  are  many.  The  bricks  in  use  on  any 
one  job  should  be  as  near  the  same  size  as  possible  as  the  extra 
mortar  filling  necessary  to  make  up  for  small  brick  detracts  from 
the  strength  of  the  sewer.  Small  brick  are  undesirable  as  the 
cost  of  laying  small  and  large  bricks  is  the  same,  but  the  thickness 
of  the  finished  sewer  is  less.  Sewer  brick  should  not  absorb 
more  than  10  to  20  per  cent  moisture  by  volume,  in  24  hours; 
except  the  special  paving  brick  used  to  prevent  erosion  at  the 
invert  which  should  absorb  less  than  5  per  cent  moisture. 

97.  Vitrified  Sewer  Block. — Blocks  and  bricks  are  manu- 
factured in  a  manner  similar  to  the  manufacture  of  vitrified  sewer 
pipe  described  in  Art.  91.  J.  M.  Egan  describes  two  types  of 
sewer  blocks 1  as  follows : 

There  are  on  the  market  two  designs  of  blocks,  one 
being  a  single-ring  block  and  the  other  a  double-ring  block. 

1  Journal  Illinois  Society  of  Engineers  for  1916,  p.  75. 


190  MATERIALS  FOR  SEWERS 

The  former  has  a  ship-lap  joint  on  the  ends  and  a  tongue- 
and-groove  joint  on  the  sides.  In  the  double  block  the  laps 
and  joints  are  made  in  the  construction  of  the  sewer  and 
the  blocks  are  placed  one  on  top  of  the  other  as  in  a  two- 
ring  brick  sewer.  The  blocks  are  hollow  longitudinally 
with  web  braces.  They  are  made  for  sewers  from  30  inches 
to  108  inches  in  diameter  and  weigh  from  40  to  120  pounds. 
They  are  18  inches  to  24  inches  long,  9  to  15  inches  wide, 
and  5  to  10  inches  thick.  Short  lengths  are  made  for 
convenience  in  construction  and  for  use  on  sharp  curves. 
Special  blocks  are  made  for  connections  and  junctions. 

A  special  block  is  also  made  for  inverts,  which  has  occasionally 
been  used  with  brick  sewers  to  avoid  the  difficulty  of  constructing 
with  brick  at  this  point.  Such  blocks  are  objectionable,  as  they 
leave  a  line  of  weakness  along  the  longitudinal  joint  so  formed. 
They  are  not  used  frequently  in  present-day  practice. 

Vitrified  blocks  are  generally  cheaper  than  bricks,  but  they 
da  not  make  so  strong  a  structure.  In  some  cases  it  is  possible  to 
lay  vitrified  block  without  the  expense  of  high-priced  bricklayers, 
thus  saving  on  the  cost  of  the  sewer  and  obtaining  a  conduit  with 
a  smoother  interior  finish. 

98.  Cast  Iron,  Steel,  and  Wood. — Cast  iron,  steel,  and  wood 
pipe  belong  more  to  the  field  of  waterworks  than  of  sewerage,  as 
they  are  not  extensively  used  in  the  construction  of  sewers. 
There  are,  however,  some  special  conditions  under  which  these 
materials  may  be  serviceable. 

The  iron  used  in  cast-iron  pipe  for  sewers,  and  in  castings  for 
manhole  covers,  inlet  frames,  etc.,  is  seldom  carefully  or  definitely 
specified.  The  standard  specifications  of  the  American  Water 
Works  Association  with  regard  to  the  quality  of  iron  for  water 
pipe  are: 

All  pipe  and  special  castings  shall  be  made  of  cast  iron 
of  good  quality  and  of  such  character  as  shall  make  the 
metal  of  the  castings  strong,  tough,  and  of  even  grain  and 
soft  enough  to  satisfactorily  admit  of  drilling  and  cutting. 
The  metal  shall  be  made  without  the  admixture  of  cinder 
iron  or  other  inferior  metal,  and  shall  be  remelted  in  a 
cupola  or  air  furnace. 

The  specifications  of  the  Sanitary  District  of  Chicago  for  the 
quality  of  iron  to  be  used  in  manhole  covers,  etc.,  are  given  on 
page  101. 


CAST  IRON,   STEEL,   AND  WOOD  191 

Although  sewer  pipes  are  not  ordinarily  subjected  to  internal 
pressure,  cast-iron  pipe  for  sewers  should  be  as  heavy  or  heavier 
than  water  pipe  to  resist  the  corrosive  action  of  the  sewage  and 
the  external  stresses  that  are  to  be  imposed  upon  it.  The  sizes 
and  details  of  standard  cast-iron  pipe  used  for  both  water  works 
and  sewerage  can  be  found  in  specification  of  the  American  and 
New  England  Water  Works  Associations. 

The  quality  of  steel  used  for  reinforcing  concrete  should  be 
carefully  specified  because  of  the  possibility  of  the  substitution  of 
inferior  material.  The  specifications  for  "  Billet  Steel  Concrete 
Reinforcement  Bars/'  of  the  American  Society  for  Testing 
Materials1  are  the  standard  for  engineering  practice,  or  the  fol- 
lowing specifications  may  be  used : 

All  reinforcement  shall  be  free  from  excessive  rust, 
scale,  paint,  or  coatings  of  any  character  which  will  tend 
to  destroy  the  bond.  The  bars  shall  be  rolled  from  new 
billets.  No  rerolled  material  will  be  accepted.  All  rein- 
forcement bars  shall  develop  an  ultimate  tensile  strength 
of  not  less  than  70,000  pounds  per  square  inch.  The  test 
specimen  shall  bend  cold  around  a  pin,  whose  diameter 
is  two  times  the  thickness  of  the  bar,  180  degrees  without 
cracking  on  the  outside  .  portion.  The  reinforcing  bars 
shall  in  all  respects  fulfill  the  requirements  of  the  standard 
specifications  of  the  American  Society  for  Testing  Materials 
for  Billet  Steel  Concrete  Reinforcing  Bars  serial  designa- 
tion A  15-14. 

The  steel  used  in  pipe  should  be  a  soft,  open-hearth  steel  with 
an  ultimate  tensile  strength  of  60,000  pounds  per  square  inch,  an 
elastic  limit  of  30,000  pounds  per  square  inch,  an  elongation  in 
8  inches  before  fracture  between  22  and  25  per  cent,  and  a  reduc- 
tion in  area  before  fracture  of  50  per  cent.  The  working  strength 
of  the  steel  is  taken  at  16,000  to  20,000  pounds  per  square  inch  in 
tension,  10,000  to  12,000  pounds  per  square  inch  in  shear,  and 
20,000  to  24,000  pounds  per  square  inch  in  bearing.  A  liberal 
allowance  should  be  made  for  corrosion.  The  standard  specifi- 
cations for  Open  Hearth  Boiler  Plate  and  Rivet  Steel  of  the  Ameri- 
can Society  for  Testing  Materials,  Aug.  16,  1919,  include  "  flange 
steel,"  which  is  suitable  for  the  manufacture  of  plates,  and  extra 
soft  steel  which  is  suitable  for  rivets. 

Steel  pipe  should  be  coated  both  inside  and  out  to  protect  it 
1  See  A.  S.  T.  M.  Standards  for  1918,  p.  148. 


192  MATERIALS  FOR  SEWERS 

against  corrosion.  The  various  proprietary  coatings  are  mainly 
coal-tar  pitches,  or  mixtures  of  coal-tar  pitch  and  asphalt.  A 
coal-tar  pitch  is  a  distillate  of  coal  tar  from  which  the  naphtha  has 
been  removed  and  to  which  about  one  per  cent  of  heavy  linseed  oil 
has  been  added.  The  coating  is  applied  to  the  pipe  at  a  tempera- 
ture of  about  300  degrees  Fahrenheit,  by  dipping  hot  pipe  in  the 
heated  coating  material.  The  pipe  should  be  carefully  cleaned 
and  all  rust  and  scale  removed  before  it  is  dipped.  In  some 
cases  the  steel  is  pickled  before  dipping.  This  consists  in  rolling 
the  cold  plates  to  a  short  radius  to  loosen  the  scale,  heating  them 
to  about  125  degrees,  and  dipping  them  in  a  warm  5  per  cent  acid 
solution  for  about  3  minutes,  and  finally  rinsing  in  a  weakly  basic 
wash  water. 

The  woods  commonly  used  for  the  manufacture  of  wood  pipe 
are  spruce,  Oregon  fir,  Douglas  fir,  and  California  redwood. 
Wood  pipe  lines  have  been  constructed  of  other  kinds  of  lumber 
but  only  in  more  or  less  unusual  conditions.  The  following  has 
been  abstracted  from  the  specifications  for  California  redwood 
given  by  J.  F.  Partridge.1 

The  staves  shall  be  of  clear,  air-dried,  California  red- 
wood, seasoned  at  least  one  year  in  the  open  air,  and  shall 
be  free  from  knots  (except  small  knots  appearing  on  one 
face  only),  sap,  dry  rot,  wind  shakes,  pitch,  pitch  seams, 
pitch  pockets,  or  other  defects  which  would  materially 
impair  their  strength  or  durability.  The  sides  of  the 
staves  shall  be  milled  to  conform  to  the  inside  and  outside 
radii  of  the  pipe;  and  the  edges  shall  be  beveled  to  true 
radial  planes.  The  staves  shall  be  milled  from  stock  sizes 
of  lumber,  the  net  finished  thickness  of  the  stave,  for  the 
various  diameters  of  pipe,  shall  be  as  given  in  Table  40. 
The  ends  shall  be  cut  square  and  slotted  to  receive  the 
metallic  tongues  which  form  the  butt  joints.  The  slots 
shall  appear  in  the  same  position  on  each  stave,  and  shall 
be  cut  to  make  a  tight  fit  with  the  tongues  in  all  directions. 
The  staves  shall  have  an  average  length  of  at  least  15  ft. 
6  in.  and  not  more  than  one  per  cent  shall  have  a  length 
of  less  than  9  ft.  6  in.  Staves  shorter  than  8  ft.  will  not 
be  accepted. 

The  bands  shall  be  spaced  on  the  pipe  with  a  factor  of 
safety  of  at  least  four,  and  shall  consist  of  round,  mild 
steel  rods,  connected  with  malleable  iron  shoes.  Either 

1  Trans.  Am.  Society  Civil  Engrs.,  Vol.  82,  1918,  p.  459. 


CAST  IRON,   STEEL,   AND  WOOD 


193 


open-hearth  or  Bessemer  steel  may  be  used.  .  .  .     The  ulti- 
mate strength  shall  be  from  55,000  to  65,000  Ib.  per  sq.  in. 

The  original  reference  should  be  consulted  for  complete  details 
and  for  specifications  for  various  kinds  of  wood  and  classes  of 
pipe.  The  discussion  following  the  specifications  is  of  value. 

Machine-made  wood  pipe  is  superior  to  stave  pipe  put  together 
in  the  field.  It  is  seldom  manufactured  in  sizes  large  enough  for 
use  in  sewers,  which  results  in  the  almost  exclusive  use  of  field 
constructed  stave  pipe.  The  steel  bands  used  to  hold  the  staves 
together  should  be  coated  similarly  to  steel  plates.  All  lumber, 
except  California  redwood  should  receive  a  preservative  coating 
of  creosote 1  or  other  material.  One  of  the  best  methods  of  pre- 
serving the  wood  is  to  keep  it  submerged  and  to  maintain  the 
pipe  under  internal  pressure. 

TABLE  40 

DETAILS  OF  DESIGN  FOB  CONTINUOUS  STAVE  WOOD  PIPE 

CLASSES  A,  B,  AND  C 
(By  J.  F.  Partridge,  Trans.  A.  S.  C.  E.,  Vol.  82,  page  461) 


Stave 

Stock 

Size 

Top  Width 

Spacing  of 

Diameter, 
Inches 

Thickness, 
Standard, 
Inches 

Size  of 
Lumber, 
Inches 

of  Band, 
Inches 

of  Staves, 
Standard, 
Inches 

Bands  for 
100  Feet 
Head 

12 

H 

2X4 

t 

3.56 

6.38 

18 

1& 

2X4 

A 

3.66 

5.76 

24 

1& 

2X4 

A 

3.70 

4.34 

30 

U 

2X6 

i 

5.48 

4.53 

36 

1A 

2X6 

J 

5.62 

3.77 

42 

if 

2X6 

i 

5.51 

3.23 

48 

if 

2X6 

£orf 

5.60 

2.  84  or  4.  41 

60 

2| 

3X6 

I 

5.56 

3.54 

72 

3| 

4X6 

lor  f 

5.69 

2.  95  or  4.  24 

84 

3* 

4X6 

i 

5.65 

3.63 

120 

3f 

4X6 

1 

5.68 

2.54 

144 

3f 

4X6 

for! 

5.64 

2.12or2.89 

!See  Trans.  Am.  Society  Civil  Eng.,  Vol.  82,  1918,  p.  482. 


CHAPTER  IX 
DESIGN  OF  THE  SEWER  RING 

99.  Stresses  in  Buried  Pipe.  —  The  stresses  which  sewer  pipe 
should  be  designed  to  resist  are:  internal  bursting  pressure,  for 
sewers  flowing  under  pressure;  stresses  due  to  handling,  for 
precast  pipe;  temperature  stresses;  and  external  loads.  The 
latter  is  by  far  the  most  important  and  frequently  is  the  only 
stress  considered  in  design. 

The  thickness  of  a  pipe  to  resist  internal  stress  should  be 


in  which  P=the  intensity  of  internal  pressure; 

R  =  the  radius  of  the  inside  of  the  pipe,  and 
ft  =  the  unit-strength  of  the  material  in  tension 

The  derivation  of  this  expression  is  simple.  The  stresses  due 
to  handling  cannot  be  computed  and  are  cared  for  by  a  thickness 
of  material  dictated  by  experience.  These  thicknesses  are  given 
for  vitrified  clay  and  cement  pipe  in  the  specifications  in  the  pre- 
ceding chapter.  Temperature  stresses  are  not  allowed  for  in  the 
design  of  the  pipe  ring,  but  allowance  must  be  made  for  them  in 
long  rigid  pipe  lines  exposed  to  wide  variations  in  temperature. 
Such  a  condition  seldom  exists  in  sewerage  works. 

The  external  forces  are  ordinarily  the  controlling  features  in 
the  design  of  sewer  rings.  The  simplest  problems  arise  in  the 
design  of  a  circular  pipe.  If  the  external  loading  is  uniform  about 
the  circumference  of  the  pipe  the  internal  stresses  will  all  be  com- 
pression. Almost  all  other  forms  of  loading  will  cause  bending 
moments  resulting  in  tension  and  compression  in  different  parts 
of  the  pipe.  The  maximum  bending  is  caused  by  two  concen- 
trated loads  diametrically  opposed.  As  such  a  condition  is 
extreme  it  is  not  cared  for  in  ordinary  design,  but  a  loading  between 

194 


DESIGN  OF  STEEL  PIPE  195 

this  condition  and  perfect  distribution  is  assumed,  as  explained 
in  Art.  103. 

100.  Design  of  Steel  Pipe. — The  stresses  which  may  occur  in 
steel  sewer  pipes  are  commonly  caused  by  the  internal  or  bursting 
pressure  of  the  contained  liquid.  Occasionally  a  steel  pipe  may 
be  used  as  a  bridge  or  as  a  stressed  member  of  a  bridge,  but  steel 
pipes  should  not  be  used  to  withstand  compression  normal  to  the 
axis.  In  order  to  avoid  such  stresses  the  bursting  tensile  stresses 
should  exceed  the  external  compressive  stresses.  Such  a  condi- 
tion in  design  requires  that  buried  pipes  shall  never  be  emptied,  a 
condition  that  cannot  always  be  fulfilled.  Precaution  should  be 
taken,  by  the  installation  of  proper  valves,  to  prevent  the  empty- 
ing of  the  pipe  at  so  rapid  a  rate  that  a  vacuum  is  created  result- 
ing in  the  collapse  of  the  pipe. 

Steel  pipes  are  ordinarily  made  of  plates  curved  to  the  proper 
diameter,  the  edges  being  held  together  by  rivets.  The  design  of 
the  pipe  consists  in  the  determination  of  the  thickness  of  the  plate 
and  the  design  of  the  riveted  joint.  The  longitudinal  joint  and 
the  thickness  of  the  plate  are  first  designed.  The  design  of  the 
joint  consists  in  determining  the  diameter  and  pitch  of  the  rivets 
and  the  thickness  of  the  plate  so  that  the  full  strength  of  the  uncut 
metal  shall  be  developed  as  nearly  as  possible  under  bearing, 
tearing,  and  shearing.  This  is  done  by  making  the  efficiency  of 
the  joint  the  same  under  all  stresses.  The  efficiency  of  the  joint 
is  the  ratio  of  the  strength  of  the  joint  under  any  kind  of  stress  to 
the  strength  in  tension  of  the  unpunched  plate.  Properties  of 
riveted  joints  are  given  in  Table  41. 

The  diameter  of  the  rivet  holes  should  be  computed  as  fs  of 
an  inch  larger  than  the  diameter  of  the  rivets.  Rivets  and  plates 
should  be  designed  for  the  nearest  or  next  largest  commercial 
size,  and  a  generous  allowance  for  corrosion  should  be  made  in 
determining  the  thickness  of  the  plate.  The  distance  from  the 
edge  of  the  plate  to  the  side  of  the  rivet  should  not  be  less  than 
1|  times  the  diameter  of  the  rivet.  The  unit-strengths  of  the 
metal  are  given  in  the  preceding  chapter. 

The  transverse  joint  must  be  designed  empirically  as  the 
stresses  in  it  are  indeterminate.  The  common  form  of  joint  for 
pipes  less  than  48  inches  in  diameter  is  a  single-riveted  lap  joint, 
and  for  larger  pipes  or  for  pipes  exposed  to  unusual  stresses,  a 
double-riveted  lap  joint  is  used.  The  same  size  rivets  are  used  as 


196 


DESIGN  OF  THE  SEWER  RING 


in  the  longitudinal  joint.     The  maximum  permissible  distance 
between  rivets  should  be  used  in  the  transverse  ioint. 

TABLE  41 

PROPERTIES  OF  RIVETED  JOINTS 
(Chicago  Bridge  and  Iron  Works) 


Type  of  Joint 

Thickness 
Plate, 
Inch 

Diameter 
of 
Rivet, 
Inch 

Pitch, 
Inches 

Effi- 
ciency 
of 
Joint, 
Per  Cent 

Thickness 
Butt 
Plate, 
Inches 

Single  riveted  lap 

i 

f 

1  88 

49 

i 

4 

f 

2.25 

50 

A 

1 

2.63 

50 

Double  riveted  lap  — 

I 

1 

2.50 

70 

A 

1 

3.00 

71 

I 

1 

3.40 

71 

Triple  riveted  lap  

i 

i 

2.39 

74 

A 

f 

2.96 

74 

t 

! 

3.53 

75 

A 

1 

4.09 

76 

Quadruple  riveted  lap  . 

I 

1 

3.20 

77 

A 

f 

3.90 

78 

Double  riveted  butt..  . 

1 

1 

3.62 

72 

1 

A 

1 

3.62 

72 

| 

f 

1 

3.62 

72 

f 

H 

1 

3.62 

72 

A 

f 

i 

4.12 

73 

A 

1 

i 

3.82 

71 

1 

i 

i 

3.48 

68 

A 

Triple  riveted  butt.... 

1 

1 

4.94 

80 

i 

f 

i 

5.62 

80 

A 

I 

i 

5.16 

78 

A 

i 

i 

4.66 

76 

A 

Quadruple  riveted  butt 

f 

i 

7.13 

84 

f 

I 

i 

6.51 

83 

H 

i 

i 

5.84 

81 

f 

DESIGN  OF  WOOD  STAVE  PIPE  197 

Pipes  used  as  compression  members  of  a  bridge  are  stiffened 
by  riveting  standard  rolled  steel  sections  longitudinally  on  the 
pipe. 

Lock  Bar  Pipe  is  a  steel  pipe  with  a  special  form  of  joint  made 
by  the  East  Jersey  Pipe  Corporation.     It  is  arranged  as  shown 
in  Fig.  78  and  has  the  ad- 
vantage  of  developing  the  HTJ — ^ 
full  strength  of   the   plate. 
It  is  equivalent  to  a   joint 
with  100  per  cent   efficien- 
cy, which  permits  the  use 
of  thinner  plates.                         .-, 

101.  Design  of  Wood 
Stave  Pipe. — In  the  de- 
sign of  wood  stave  pipe1 
the  entire  bursting  pres-  Fro.  78.— Lock  Bar  Pipe. 

sure   is  taken  up  by  steel 

bands  wrapped  around  the  outside  of  wood  staves  which  make 
up  the  shell  of  the  pipe.  The  pipe  is  not  designed  to  resist  external 
loads  except  those  which  may  be  overcome  by  the  internal  pressure 
in  the  pipe.  The  thickness  of  the  staves  is  fixed  by  experience. 
The  sizes  of  staves  and  bands  recommended  by  J.  F.  Partridge  2 
are  given  in  Table  40.  The  size  of  the  steel  bands  can  be  deter- 
mined from  the  expression; 

S  =  Cr(R+t) 

in  which     S  =  the  total  stress  in  the  band; 

R  =  the  radius  of  the  inside  of  the  pipe; 
2  =  the  thickness  of  the  stave; 

r=the  area  of  bearing  per  unit  length  of  the  band  on 
the  wood.  For  circular  bands  it  is  assumed  as 
the  radius  of  the  band; 

C  =  the  crushing  strength  of  wood,  usually  taken  at 
650  pounds  per  sq.  in. 

The  preceding  expression  can  be  derived  easily  by  the 
application  of  the  laws  of  mechanics,  and  from  it  the 

1  See  Trans.  Am.  Society  Civil  Engr.,  Vol.  41,  1899,  p.  76,  and  Vol.  82, 
1918,  p.  433,  Eng.  News,  Vol.  74,  1915,  p.  400,  and  Vol.  75,  1916,  p.  911. 

2  Trans.  Am.  Soc.  Civil  Engrs.,  Vol.  82,  1918,  p.  433. 


198 


DESIGN  OF  THE  SEWER  RING 


expression    for    the   distance    between   bands   follows   logically. 
It  is, 

S 
P    PR+kt 

in  which     S  =  the  strength  of  the  band ; 

p  =  the  distance  between  bands; 
P  =  the  intensity  of  bursting  pressure  in  the  pipe; 
R  =  the  radius,  of  the  inside  of  the  pipe; 
t  =  ihe  thickness  of  the  staves; 

fc  =  the  swelling  strength  of  wood,  usually  taken  at 
100  pounds  per  sq.  in. 

Transverse   joints   between   staves   are    closed    by   inserting 

metal  strips  between  them,  or  by  shaping,  the  edges  irregularly 

so  that  they  fit  closely  together 
with  an  irregular  joint.  Trans- 
verse joints  between  all  staves 
at  any  one  point  are  avoided  by 
splitting  the  joints  between  staves. 
Longitudinal  j  oints  between 
staves  are  usually  made  smooth 

FIG.  79.— Shoe  for  Wood  Stave  Pipe,  and  are  closed  by  steel  bands 

which  are  drawn  tight  about  the 

pipe  by  inserting  the  ends  in  coupling  shoes  as  shown  in  Fig.  79. 
102.  External  Loads  on  Buried  Pipe. — Prof.  Anston  Marston 

and  H.  C.  Anderson  published l  the  results  of  a  series  of  experiments 

on  the   loads   on   buried  pipes  which  are 

of  extreme  value   in   the  design  of  sewer 

pipe.     The  load  on  the  pipe  is  given  by 

the    empirical    expression    W=CwB2,    in 

which  w  is  the  weight  of   the   backfilling 

material  in   pounds   per   cubic  foot,  B  is 

the  width  of   the  trench  in  feet   at   the 

elevation  of  the  end   of  a  radius  making 

an   angle  of  45  degrees  upwards  with  the 

horizontal  diameter  of  the  pipe  as  illus- 
trated in  Fig.  80,  and  C  is  a  coefficient 

dependent  on  the  character  of  the  backfill  and  the  ratio  of  the 

bulletin  No.  31  of  the  Engineering  Experiment  Station  of  the  Iowa 
State  College  of  Agriculture. 


FIG.  80.— B  in  Formula 


EXTERNAL  LOADS  ON  BURIED  PIPE 


199 


width  to  the  depth  of  the  trench.  Values  of  C  are  given  in 
Table  42.  The  weights  of  various  classes  of  backfilling  are  given 
in  Table  43. 

TABLE  42 

APPROXIMATE  SAFE  WORKING  VALUES  OP  C  IN  THE  EXPRESSION 

W  =  CwBz 

From  Bulletin  No.  31  of  the  Engineering  Experiment  Station,  Iowa  State 
College  of  Agriculture. 


Approximate  Values  of  C 

Approximate  Values  of  C 

Ratio 
of 
Depth 
to 
Width 

Damp 
Top  Soil 
and  Dry 
and  Wet 
Sand    - 

Satu- 
rated 
Top 
Soil 

Damp 
Yellow 
Clay 

Satu- 
rated 
Yellow 
Clay 

Ratio 
of 
Depth 
to 
Width 

Damp 
Top  Soil 
and  Dry 
and  Wet 
Sand 

Satu- 
rated 
Top 
Soil 

Damp 
Yellow 
Clay 

Satu- 
rated 
Yellow 
Clay 

0.5 

0.46 

0.47 

0.47 

0.48 

7.0 

2.73 

2.95 

3.19 

3.55 

1.0 

0.85 

0.86 

0.88 

0.90 

7.5 

2.78 

3.01 

3.27 

3.65 

1.5 

1.18 

1.21 

1.25 

1.27 

8.0 

2.82 

3.06 

3.33 

3.74 

2.0 

1.47 

1.51 

1.56 

1.62 

8.5 

2.85 

3.10 

3.39 

3.82 

2.5 

1.70 

1.77 

1.83 

1.91 

9.0 

2.88 

3.14 

3.44 

3.89 

3.0 

1.90 

1.99 

2.08 

2.19 

9.5 

2.90 

3.18 

3.48 

3.96 

3.5 

2.08 

2.18 

2.28 

2.43 

10.0 

2.92 

3.20 

3.52 

4.01 

4.0 

2.22 

2.35 

2.47 

2.65 

11.0 

2.95 

3.25 

3.58 

4.11 

4.5 

2.34 

2.49 

2.63 

2.85 

12.0 

2.97 

3.28 

3.63 

4.19 

5.0 

2.45 

2.61 

2.78 

3.02 

13.0 

2.99 

3.31 

3.67 

4.25 

5.5 

2.54 

2.72 

2.90 

3.18 

14.0 

3.00 

3.33 

3.70 

4.30 

6.0 

2.61 

2.81 

3.01 

3.32 

15.0 

3.01 

3.34 

3.72 

4.34 

6.5 

2.68 

2.89 

3.11 

3.44 

00 

3.03 

3.38 

3.79 

4.50 

TABLE  43 

APPROXIMATE  WEIGHTS  OF  DITCH  FILLING  MATERIAL  TO  BE  USED  IN  THE 
EXPRESSION     W  =  CwB2  * 


Ditch  Filling 


Pounds  per  Cubic  Foot 


Partly  compacted  top  soil  (damp) .  . 

Saturated  top  soil 

Partly  compacted  damp  yellow  clay 

Saturated  yellow  clay 

Dry  sand 

Wet  sand..  


90 
110 
100 
130 
100 
120 


*  From  bulletin  No.  31,  Engineering  Experiment  Station,  Iowa  State  College  of  Agri- 
culture. 


200 


DESIGN  OF  THE  SEWER  RING 


Where  surface  loads  are  to  be  carried  on  the  sewer  trench  the 
proper  proportion  of  the  load  to  be  carried  by  the  sewer  is  deter- 
mined by  the  expression  LP  =  CL,  in  which  Lp  is  the  equivalent 
backfill  load  per  unit  length  of  the  trench,  L  i;  the  surface  load 
per  unit  length  of  the  trench,  and  C  is  a  coefficient  in  which  allow- 
ance is  made  for  the  character  of  the  backfilling,  the  ratio  of  depth 
to  width  o~  trench,  and  the  character  of  the  load,  whether  long  or 
short.  A  long  load  *s  a  load  extending  along  the  length  of  the 
trench  such  as  a  pile  of  building  material.  A  short  load  is  one 
extend  ng  across  the  trench  and  for  only  a  short  distance  along  it, 
such  as  that  caused  by  a  street  car  or  road  roller  crossing  the  trench. 
Values  of  C  are  given  in  Table  44  for  long  loads,  and  in  Table  45 
for  short  loads.  Values  of  long  and  short  loads  occasionally  met 
in  practice  are  given  in  Tables  46  and  47  respectively, 

TABLE  44 
RATIO  OF  LOAD  ON  PIPE  TO  LONG  LOAD  ON  TRENCH  * 


Ratio 
of 
Depth 
to 
Width 

Sand 
and 
Damp 
Top 
Soil 

Satu- 
rated 
Top 
Soil 

Damp 
Yellow 
Clay 

Satu- 
rated 
Yellow 
Clay 

Ratio 
of 
Depth 
to 
Width 

Sand 
and 
Damp 
Top 
Soil 

Satu- 
rated 
Top 
Soil 

Damp 
Yellow 
Clay 

Satu- 
rated 
Yellow 
Clay 

0.0 

1.00 

1.00 

1.00 

1.00 

3.0 

0.37 

0.41 

0.45 

0.51 

0.5 

0.85 

0.86 

0.88 

0.89 

4.0 

0.27 

0.31 

0.35 

0.41 

1.0 

0.72 

0.75 

0.77 

0.80 

5.0 

0.19 

0.23 

0.27 

0.33 

1.5 

0.61 

0.64 

0.67 

0.72 

6.0 

0.14 

0.17 

0.20 

0.26 

2.0 

0.52 

0.53 

0.59 

0.64 

8.0 

0.07 

0.09 

0.12 

0.17 

2.5 

0.44 

0.43 

0.52 

0.57 

10.0 

0.04 

0.05 

0.07 

0.11 

*  From  Bulletin  No.  31,  Engineering  Experiment  Station,  Iowa  State  College  of  Agri- 
culture. 

For  example,  let  it  be  desired  to  determine  the  load 
on  a  72-inch  concrete  sewer  with  a  9-inch  shell  under  the 
following  conditions:  depth  of  backfill  over  the  top  of 
the  pipe,  15  feet;  character  of  backfill,  saturated  yellow 
clay;  superimposed  load,  pile  of  building  brick  6  feet 
high.  The  ratio  of  the  depth  of  backfill  to  the  width  of 
the  trench  is  15 -i- 9  or  1.67.  The  coefficient  in  the  expression 
CwB2  is  1.39,  from  Table  42.  The  weight  of  saturated 
yellow  clay  is  130  pounds  per  cubic  foot,  from  Table  43. 
Therefore  the  load  per  foot  length  of  the  sewer  due  to  the 
backfill  is: 


W  =  CwB2  =  1 .39  X 130  X  81  =  14,600  pounds. 


EXTERNAL  LOADS  ON  BURIED  PIPE 


201 


TABLE  45 
RATIO  OF  LOAD  ON  PIPE  TO  SHORT  LOAD  ON  TRENCH 


Ratio 
of 
Height 
to 
Width 
of 
Trench 

Sand  and  Damp 
Top  Soil 

Saturated 
Top  Soil 

Damp                Saturated 
Yellow  Clay         Yellow  Clay 

Length  of  Load  Equal  to 

Width 
of 
Trench 

A 

Width 
of 
Trench 

Width 
of 
Trench 

TV 
Width 
of 
Trench 

Width 
of 
Trench 

A 

Width 
of 
Trench 

Width 
of 
Trench 

TV 

Width 
of 
Trench 

0.0 
0.5 
1.0 
1.5 
2.0 
2.5 
3.0 
4.0 
5.0 
6.0 
8.0 
10.0 

1.00 
0.77 
0.59 
0.46 
0.35 
0.27 
0.21 
0.12 
0.07 
0.04 
0.02 
0.01 

1.00 
0.12 
0.02 

1.00 

0.78 
0.61 
0.48 

1.00 
0.13 
0.02 

1.00 
0.79 
0.63 
0.51 

1.00 
0.13 
0.02 

1.00 
0.81 
0.66 
0  54 

1.00 
0.13 
0.02 



0.38 
0.29 
0.23 
0  12 

0.40 

0.44 

0.32 
0.25 
0.16 
0.10 
0.06 
0.03 
0.01 



0.35 
0.29 
0.19 
0.13 
0.08 
0.04 
0  02 

0.09 
0.05 
0.02 
0.01 



*  From  Bulletin  No.  31,  Engineering  Experiment  Station,  Iowa  State  College  of  Agri 
culture. 


TABLE  46 

WEIGHTS  OF  COMMON   BUILDING  MATERIAL  WHEN  PILED  FOR  STORAGE. 
POUNDS  PER  CUBIC  FOOT 


Brick 120 

Cement 90 

Sand 90 

Broken  stone .  .  .150 


Lumber 35 

Granite  paving 160 

Coal 50 

Pig  iron 400 


The  pressure  of  the  pile  of  brick  per  square  foot  of  trench 
area  is,  from  Table  46,  120X6  =  720  pounds  per  square 
foot.  The  value  of  C  from  Table  44,  is  about  0.70.  There- 
fore LP  is  0.7X9X720=4536  pounds.  The  equivalent 
depth  of  backfill  weighing  130  pounds  per  cubic  foot  is 

=3.88  foot.    The  total  equivalent  depth,  of  back- 


202 


DESIGN  OF  THE  SEWER  RING 


fill  is  therefore  3.88+15  =  18.88  feet.     The  ratio  of  depth 

18  88 
to  width  is  — ^-   =2.98.     The  coefficient  C  in  the  expression 

W  =  CwB2   is   2.17.     The   total   load   per  foot   length   of 
sewer  is  therefore  W  =  2. 17X130X81  =  22,800  pounds. 

TABLE  47 

WEIGHTS  OF  SHORT  LOADS  ON  SEWER  TRENCHES 
(Adapted  from  Specifications  of  the  American  Bridge  Company  for  Bridges) 


Street  railways,  heavy 

Street  railways,  light 

For  city  streets,  heavy  traffic 

For  city  streets,  moderate  traffic.. .  . 

For  city  streets,  light  traffic  or  coun- 
try, roads 


Road  rollers , 


A  load  of  24  tons  on  2  axles  on  10  foot 

centers. 
A  load  of  18  tons  on  2  axles  on  10  foot 

centers. 
A  load  of  24  tons  on  2  axles  10  feet 

apart  and  5  foot  gage. 
A  load  of  12  tons  on  2  axles  10  feet 

apart  and  5  foot  gage. 
A  load  of  6  tons  on  2  axles  10  feet 

apart  and  5  foot  gage. 

Total  weight  30,000  pounds.  Weight 
on  front  wheel,  12,000  pounds,  and 
on  each  of  two  rear  wheels,  9,000 
pounds.  Width  of  front  wheel, 
4  feet  and  of  each  of  two  rear  wheels 
20  inches.  Distance  between  front 
and  rear  axles  11  feet.  Gage  of 
rear  wheels,  5  feet,  c.  to  c. 


103.  Stresses  in  Circular  Ring — In  Fig.  8  la  the  loads  shown 
indicate  the  distribution  ordinarily  assumed  in  sewer  design,  the 
forces  being  uniformly  distributed  across  the  diameter.  To  find 
the  bending  moment  in  the  pipe  caused  by  this  loading,  let  ah  in 
Fig.  816  represent  a  section  of  a  pipe  loaded  with  equally  dis- 
tributed horizontal  and  vertical  forces.  Then  the  vertical  com- 
ponent on  a  strip  of  differential  length  ds  is  wds  cos  6  and  the 
horizontal  component  is  wds  sin  6  and  resolving,  the  resultant 
normal  to  the  surface  is  wds,  in  which  w  is  the  intensity  per  unit 
length  of  the  horizontal  and  vertical  forces  and  6  is  the  angle 
which  the  tangent  to  ds  makes  with  the  horizontal.  Thus  the  load- 
ing of  the  nature  shown  in  Fig.  816  is  equivalent  to  a  loading  of 
equally  distributed  normal  forces  which  give  no  moment  in  the  ring. 


STRESSES  IN  CIRCULAR  RING 


203 


Considering  a  ring  subjected  to  vertical  forces  only,  the 
moments  will  be  as  shown  in  Fig.  Sic  and  if  loaded  with  horizontal 
forces  only,  the  moments  will  be  as  shown  in  Fig.  Sid.  Because 
of  the  symmetry  of  the  figure,  moment  (1)  equals  moment  (4) 
but  is  opposite  in  direction  and  moment  (2)  equals  moment  (3) 
but  is  opposite  in  direction.  When  the  horizontal  and  vertical 
forces  are  combined  on  the  same  ring  as  in  Fig.  816  these  moments 
cancel  each  other  as  has  been  proven.  Therefore  moment  (1) 
equals  moment  (2)  and  moment  (3)  equals  moment  (4).  Then 
in  Fig.  Sle,  Ma  =  Mb.  NowSM  =  Ofor  conditions  of  equilibrium, 

therefore  M«+Jfi+(W)-0   and    solving   Ma  =  -r.     This 


moment  occurs  at  the  ends  of  the  horizontal  and  vertical  diameters 
and  causes  tension  on  the  inside  of  the  pipe  at  the  top  and  on  the 


-b- 


-d- 


-e- 


FIG.  81. — Distribution  of  Stresses  on  Buried  Pipe. 


outside  at  the  ends  of  the  horizontal  diameter.  There  will  also 
be  compression  at  each  end  of  the  horizontal  diameter  equal  to 
one-half  of  the  total  load  on  the  pipe.  If  the  material  of  the 
pipe  is  homogeneous,  the  maximum  fiber  stress  /  can  be  found 

My     P 
through  the  expression  /=-y-±-j-  in  which  M  is  the  bending 

moment,  y  is  the  distance  from  the  neutral  axis  to  the  extreme 
fiber  of  a  cross-section  of  the  shell  of  the  pipe  of  unit  length,  /  is  the 
moment  of  inertia  of  this  cross-section  about  its  neutral  axis,  P  is 
one-half  the  total  load  on  the  pipe,  and  A  is  the  area  of  the  cross- 
section.  For  reinforced  concrete,  the  standard  formulas  should 
be  used  with  this  expression  for  M.  The  stresses  in  a  circular 
ring  subjected  to  other  distributions  of  loads  are  shown  in  Table 
48.  An  exhaustive  study  of  the  stresses  in  circular  rings  was 
published  by  Prof.  A.  N.  Talbot  in  Bulletin  No.  22  of  the  Engi- 
neering Experiment  Station  at  the  University  of  Illinois,  1908. 


204 


DESIGN  OF  THE  SEWER  RING 


TABLE  48 

MAXIMUM  STRESS  IN  FLEXIBLE  RINGS  DUE  TO  DIFFERENT  LOADINGS 
(From  Marston) 


Symmetrical 
Vertical  Loadings 

Moment 
at  Crown 
of  Sewer 

Moment 
at  End  of 
Horizontal 
Diameter 

Com- 
pressive 
Thrust 
at 
Crown 

Com- 
pressive 
Thrust 
at  End  of 
Horizontal 
Diameter 

Shear 
at 
Crown 

Shear 
at  End  of 
Horizontal 
Diameter 

Character 

Width 

Concentrated. 
Uniform  
Uniform 

0° 
60° 
90° 
180° 

+  .318/2— 
12 

W 

+  .  207  R  — 
12 

W 
+  .169*- 

+-125V 

W 
-  .  182R  
12 

W 
-  .  168R  — 
12 

W 

-  .  i54/e  — 

12 

W 
-  .  125R  
12 

0.000 
0.000 
0.000 
0.000 

W 
+  .500^ 

W 

+  -5CV 

W 
+  .50V 

+  .500^ 
12 

W 
O.SOOjj 

W 

o.ooo  — 

12 

W 
0.000- 

w 

o.ooo  — 

12 

0.000 
0.000 
0.000 
0.000 

Uniform  

R  =  the  radius  of  the  pipe,  W  =  total  weight  of  ditch  filling  and  superimposed  load  plus 
|  of  the  weight  of  the  pipe  itself  (usually  neglected),  expressed  in  pounds  per  foot  length  of 
pipe.  Moments  are  inch-pounds  per  inch  length  of  pipe.  Shears  and  thrusts  are  in  pounds 
per  inch  length  of  pipe. 

104.  Analysis  of  Sewer  Arches. — The  preceding  method  for 
the  determination  of  the  stresses  in  a  sewer  ring  has  referred  only 
to  a  circular  pipe  uniformly  loaded.  Other  methods  must  be 
used  if  the  pipe  is  not  circular  or  the  load  is  not  uniformly  dis- 
tributed. The  simplest  method,  is  the  static  or  so-called  vouis- 
soir  method.  In  this  method  the  arch  is  assumed  to  be  fixed  at 
both  ends,  presumably  at  the  springing  line  or  line  of  intersection 
between  the  inside  face  of  the  arch  and  the  abutment,  and  it  is  so 
designed  that  the  resultant  of  all  the  forces  acting  on  any  section 
shall  lie  within  the  middle  third  of  that  section. 

To  design  an  unreinforced  sewer  arch  by  the  vouissoir  method, 
a  desired  arch  is  drawn  to  scale  in  apparently  good  proportions 
for  the  loadings  anticipated.  The  arch  is  then  divided  into  any 
number  of  sections  of  equal  or  approximately  equal  length  called 
vouissoirs,  and  the  line  of  action  of  the  resultant  load,  including 
the  weight  of  the  vouissoir  is  drawn  above  each  vouissoir  as  shown 
in  Fig.  82.  The  forces  are  assumed  to  act  as  shown  in  the  figure. 
In  symmetrically  loaded  sewer  arches  there  is  no  vertical  reaction 
at  the  crown.  The  resultant  R  is  assumed  to  act  at  the  lower 
middle  third  of  the  skewback,  which  is  the  inclined  joint  between 
the  arch  and  the  abutment.  The  upper  horizontal  force  H  is 
assumed  to  act  at  the  upper  middle  third  of  the  middle  or  crown 


ANALYSIS  OF  SEWER  ARCHES 


205 


section.  The  magnitude  of  H  is  computed  by  equating  the  sum 
of  the  moments  of  all  forces  about  the  point  of  application  of  R 
at  the  skewback  to  zero,  and  solving.  The  force  polygon  is  then 
drawn  as  shown  in  Fig.  83,  and  the  equilibrium  polygon  is  com- 
pleted in  Fig.  82  with  its  rays  parallel  to  the  corresponding  strings 
drawn  from  the  end  of  H  as  origin  in  Fig.  83.  If  the  equilibrium 
polygon  line,  called  the  resistance  line,  lies  wholly  within  the 
middle  third  of  each  vouissoir,  the  arch  is  satisfactory  to  support 
the  assumed  load  without  reinforcement.  If  any  portion  of  the 
resistance  line  lies  outside  of  the  middle  third,  an  attempt  should 
be  made  to  find  a  resistance  line  which  lies  wholly  within  the 
middle  third,  The  true  resistance  line  is  that  which  deviates  the 


R     v 

FIG.  82. — Voussoir  Arch  Analysis.  FIG.  83. — Force  Polygon  for 

Voussoir  Arch  Analysis. 

least  from  the  neutral  axis  of  the  arch.  To  approximate  more 
nearly  the  true  resistance  line  find  two  points  at  which  the  resist- 
ance line  already  drawn  deviates  the  most  from  the  neutral  axis 
of  the  arch.  Select  points  M  and  N  on  these  joints,  M  being 
nearer  the  crown  than  N.  Then  let  W\  and  W2  be  the  sum  of  all 
the  loads  between  the  crown  and  M  and  N  respectively,  y  repre- 
sent the  vertical  distance  from  the  crown  to  N,  and  y'  represent 
the  vertical  distance  between  M  and  N,  and  x\  and  X2  represent 
the  horizontal  distance  from  W\  and  W2  to  M  and  N  respectively. 
Then  the  horizontal  thrust  H,  and  a,  the  distance  from  the  crown 
to  the  point  of  application  of  H,  are, 


y' 

W2x2  t 
a=y--fr^ 

1  From  Vouissoir  Arches  by  Cain. 


206 


DESIGN  OF  THE  SEWER  RING 


A  resistance  line  should  be  drawn  with  this  new  horizontal  thrust. 
If  no  resistance  line  can  be  found  lying  wholly  within  the  middle 
third,  new  sections  should  be  designed  until  a  resistance  line  can  be 
drawn  lying  wholly  within  the  middle  third — unless  the  arch  is  to 
be  reinforced.  A  number  of  satisfactory  arches  should  be  designed 
and  the  easiest  one  to  build  should  be  selected.  This  method  is 
limited  in  its  application  to  sewer  arches  with  rigid  side  walls  and 
it  cannot  be  extended  to  include  the  invert.  Although  an  approxi- 
mate method  it  is  accurate  within  less  than  10  per  cent  of  the  true 
stresses  and  is  usually  quite  close. 

The  elastic  method  for  the  design  of  arches  locates  the  true 
line  of  resistance  without  approximations  and  is  more  accurate 
though  not  so  simple  to  apply  as  the  static  or  vouissoir  method. 
In  this  method  a  desired  form  of  arch  is  drawn  as  in  the  static- 
method  and  subdivided  into  vouissoirs  so  that  the  distance  S 
along  the  neutral  axis  between  joints  is  such  that  the  ratio  I/S 
shall  be  the  same  for  all  vouissoirs.  /  is  the  average  of  the 
moments  of  inertia  of  the  surfaces  of  the  two  limiting  joints  about 
the  neutral  axis.  If  the  thickness  of  the  arch  is  constant  the 
distance  between  joints  will  be  the  same.  The  method  for  divid- 
ing the  arch  into  sections  such  that  the  ratio  I/S  shall  be  a  con- 
stant l  is  as  follows:  divide  the  half  arch  axis  into  any  number  of 


FIG.  84. — Method  for  Dividing  Arch  into  Proportion  I/S. 

equal  parts;  measure  the  radial  depth  at  each  point  of  division; 
lay  off  the  length  of  the  arch  axis  to  scale  on  a  straight  line; 
divide  this  line  into  the  same  number  of  equal  parts  as  the  half 
arch,  as  shown  in  Fig.  84;  at  each  point  erect  a  perpendicular 
1  Baker's  Masonry,  10th  Edition,  p.  676. 


ANALYSIS   OF  SEWER  ARCHES 


207 


equal  in  length  by  scale  to  the  moment  of  inertia  at  the  correspond- 
ing point  on  the  arch  section;  draw  a  smooth  curve  through  the 
tops  of  these  lines;  draw  a  line  ab  at  any  slope  from  the  center  of 
the  original  straight  line  to  the  curve,  and  then  a  line  be  back  to 
the  straight  line  to  form  an  isosceles  triangle  abc;  continue  forming 
these  triangles  in  a  similar  manner  thus  dividing  the  original 
straight  line  in  the  required  ratio.  The  distance  between  joints 
is  represented  by  the  bases  of  the  triangles.  By  construction  the 
altitude  of  the  triangle  represents  the  average  moment  of  inertia 
between  the  two  limiting  joints.  The  base  of  each  isosceles 
triangle  is  S,  and  I/S  =  %  tan  a  in  which  a  is  the  base  angle  of  all 
the  isosceles  triangles. 


FIG.  85. — Elastic  Arch  Analysis. 


The  following  steps  in  the  procedure  are  taken  from  the  second 
edition  of  the  American  Civil  Engineers  Pocket  Book,  p.  634: 

In  Fig.  85  let  the  middle  points  of  the  joints  be  marked 
1,  2,  3,  etc.  and  the  coordinates  x  and  y  from  the  crown 
be  found  for  each  by  computation  or  measurement.  For 
a  load  W  placed  at  one  of  these  points,  let  z  denote  the 
distance  from  it,  toward  the  nearest  skewback,  to  another 
middle  point.  Let  l^zx  be  the  sum  of  the  products  of  all 
the  values  of  z  by  the  corresponding  x,  and  2«y  be  the  sum 
of  all  the  products  of  z  by  the  corresponding  y;  that  is, 
each  z  in  the  last  two  summations  is  multiplied  by  the  x 
or  y  of  the  point  back  of  W  which  corresponds  to  z. 

For  a  single  load  W  on  the  left  semi-arch  of  Fig.  85 
the  following  formulas  are  deduced  from  the  elastic  theory, 


208  DESIGN  OF  THE  SEWER  RING 

n  being  the  number  of  parts  into  which  the  semi-arch  is 
divided. 


n  TT 

Horizontal  thrust,     H=   -    n_y?  .    •     (1) 

Moment  at  Crown,  MO  =  —  —     ...    (2) 


Shear  at  Crown,       ^0  =  .....     (3) 


For  symmetrical  loading  such  as  W  on  the  left  and  W  on 
the  right  the  horizontal  thrust  and  crown  moment  due 
to  both  loads  are  double  those  found  by  the  above  formulas, 
while  the  crown  shear  VQ  is  zero.  For  several  loads  unsym- 
metrically  placed  the  formulas  are  to  be  applied  to  each 
in  succession  and  the  results  added  algebraically,  the 
value  of  Fo  being  taken  as  negative  for  the  left  semi-arch 
and  positive  for  the  right  semi-arch. 

For  any  joint  whose  middle  point  is  at  a  distance  x 
from  the  crown 

M  =  M0+Hy+V0x-'2Wz, 
V=V0-2W, 

where  2TF  is  the  sum  of  all  the  loads  between  the  joint 
and  the  crown  and  2Wz  is  the  sum  of  the  moments  of 
those  loads  with  respect  to  the  middle  of  the  joint.  The 
components  of  the  resultant  thrust  normal  and  parallel 
to  the  joints  are, 

N  =  H  cos  0—  Fsin  0, 

F  =  Hsm6+VcosO, 

in  which  6  is  the  angle  which  the  plane  of  the  joint  makes 
with  the  vertical. 

The  distances  from  the  neutral  axis  to  the  resistance 
line  are, 

at  the  crown,  eo  =  ^r, 
n. 

at  the  j  oint,     e  =  •=•=-. 

The  resistance  line  should  be  located  as  in  the  vouissoir 
method  and  if  not  within  the  middle  third  a  new  design  should  be 
studied. 


REINFORCED   CONCRETE  SEWER  DESIGN  209 

105.  Reinforced  Concrete  Sewer  Design.— The  method  to  be 
followed  in  the  design  of  reinforced  concrete  arches  is  similar 
except  that  the  moment  of  inertia  should  include  both  the  con- 
crete and  the  steel,  that  is, 

I  =  Ic+nI3, 

in  which  7  is  the  moment  of  inertia  to  be  employed,  Ic  is  the 
moment  of  inertia  of  the  concrete,  Is  is  the  moment  of  inertia  of 
the  steel,  and  n  is  the  ratio  of  their  moduli  of  elasticity,  generally 
taken  as  15.  All  of  the  moments  of  inertia  are  referred  to  the 
neutral  axis  of  the  beam.  The  reinforcement  called  for  in  pre- 
cast circular  pipes  is  given  in  Table  39.  Sewers  cast  in  place  are 
ordinarily  designed  to  avoid  reinforcement,  except  where  the 
depth  of  cover  is  small  and  the  sewer  may  be  subjected  to  super- 
imposed loads. 

Concrete  sewers  are  sometimes  reinforced  longitudinally,  with 
expansion  joints  from  30  to  50  feet  apart.  This  reinforcement  is 
to  reduce  the  size  of  expansion  and  contraction  cracks  by  dis- 
tributing them  over  the  length  of  a  section.  The  pipe  is  divided 
into  sections  to  concentrate  motion  due  to  expansion  or  contrac- 
tion at  definite  points  where  it  can  be  cared  for. 

The  amount  of  longitudinal  reinforcement  to  be  used  is  a 
matter  of  judgment.  It  varies  in  practice  from  0.1  to  0.4  per 
cent  of  the  area  of  the  section.  Since  the  coefficients  of  expan- 
sion of  concrete  and  of  steel  are  nearly  the  same,  movements  of 
the  structure  are  as  important  as  the  stresses  due  to  changes  in 
temperature. 

Because  of  the  uncertain  and  difficult  conditions  under  which 
concrete  sewers  are  frequently  constructed  it  is  advisable  to 
specify  the  best  grade  of  concrete  and  not  to  stress  the  concrete 
over  450  pounds  per  square  inch  in  compression,  with  no  allowable 
stress  in  tension.  The  concrete  covering  of  reinforcing  steel 
should  be  thicker  than  is  ordinarily  used  for  concrete  building 
design,  because  of  the  possibility  of  poor  concrete  allowing  the 
sewage  to  gain  access  to  the  steel,  resulting  in  more  rapid  deteriora- 
tion than  would  be  caused  by  exposure  to  the  atmosphere.  A 
minimum  covering  of  about  2  inches  is  advisable,  except  in  very- 
thin  sections  not  in  contact  with  the  sewage.  A  minimum  thick- 
ness of  concrete  of  about  9  inches  is  frequently  used  in  design, 
although  crown  thicknesses  of  4J  inches  have  been  used  with 


210  DESIGN  OF  THE  SEWER  RING 

success.    Greater  thicknesses  should  be  used  near  the  surface, 
particularly  in  locations  subjected  to  heavy  or  moving  loads. 

Brick  linings  are  often  provided  for  the  invert  where  moder- 
ately high  velocities  of  about  10  feet  per  second  when  flowing  full 
are  to  be  expected.  For  velocities  in  the  neighborhood  of  20  feet 
per  second  the  invert  should  be  lined  with  the  best  quality  vitri- 
fied brick.  Although  concrete  may  erode  no  faster  than  brick 
under  the  same  conditions,  brick  linings  are  more  easily  replaced 
and  at  a  smaller  expense. 


CHAPTER  X 
CONTRACTS  AND  SPECIFICATIONS 

106.  Importance  of  the  Subject. — Sewers  may  be  constructed 
by  day  labor  or  by  contract.     Under  the   day  labor  plan  a  city 
official  or  commission  is  charged  with  the  purchase  of  material, 
the  hiring  and  firing  of  employees,  and  the  management  of  the 
work.     Under  the  contract  system  a  private  individual  or  com- 
pany contracts  to  supply  all  the  material  and  labor  necessary  for 
the  completion  of  the  work. 

Under  the  day  labor  plan  all  persons  engaged  are  "  working 
for  the  City."  There  is  not  the  same  sense  of  individual  responsi- 
bility, the  same  incentive  to  economize,  the  same  feeling  of  loyalty 
that  is  inspired  by  work  under  the  personality  of  a  contractor. 
Under  either  the  day  labor  or  contract  plan  unscrupulous  politics 
are  likely  to  enter  into  the  relations  of  the  employees  of  the  city 
and  the  city  officials  or  between  the  contractor  and  the  city 
officials.  Neither  the  day  labor  nor  the  contract  plan  offer  a  sure 
cure  for  unscrupulous  political  misdealings.  Under  the  contract 
plan  the  contractor  is  led  to  keep  his  bid  as  low  as  possible,  realiz- 
ing the  competition  of  other  bidders,  and  during  construction  he 
will  obtain  greater  efficiency  from  his  labor  because  of  their 
realization  of  the  different  conditions  under  which  they  are  work- 
ing. In  some  states  and  cities  it  is  illegal  for  the  municipality  to 
do  sewer  construction  except  under  the  contract  method. 

The  contract  method  is  therefore  used  in  the  majority  of 
cases,  and  it  is  to  the  interest  of  the  engineer  that  he  be  acquainted 
with  the  essentials  of  contracts  and  specifications  necessary  for 
the  proper  prosecution  of  sewer  construction. 

107.  Scope  of  Subject. — The  making  of  a  contract  is  one  of 
the  most  common  episodes  of  every  day  life.     The  contract  may 
be  an  informal  verbal  agreement  to  meet  at  a  certain  place  at  a 
certain  time,  or  it  may  be  a  formal  document  hedged  about  by 
confusing  legal  phraseology  and  bearing  varieties  of  penalties  and 

211 


212  CONTRACTS  AND  SPECIFICATIONS 

dire  consequences  in  the  event  of  its  breach.  The  purpose  of  this 
chapter  is  to  explain  only  those  general  features  of  an  engineering 
contract  which  have  particular  bearing  upon  sewerage  construc- 
tion. Only  the  most  essential  points  can  be  touched  in  the  limited 
space  available  to  this  subject,  it  being  presumed  that  the  engi- 
neer is  previously  grounded  in  the  principles  of  business  law.1 

108.  Types  of  Contracts. — Contracts  are  known  as  lump  sum, 
cost-plus,  unit-price,  and  by  other  titles  indicating  the  method  of 
payment. 

A  lump  sum  contract  is  one  in  which  a  stated  amount  is  fixed 
upon,  before  the  execution  of  the  contract,  to  be  paid  for  all  the 
work  to  be  done  and  materials  to  be  furnished  under  the  contract. 
Such  an  arrangement  is  not  advisable  for  a  sewer  contract,  as  the 
cautious  contractor  will  bid  high  enough  to  protect  himself  in  the 
event  of  any  probable  emergency.  The  principal  must  therefore 
pay  whether  the  emergency  or  unforeseen  difficulty  is  met  or  not. 
The  advantage  of  this  type  of  payment  is  that  the  principal 
knows  exactly  the  cost  of  the  work  to  him  before  construction  is 
commenced. 

Cost-plus  contracts  are  those  in  which  the  cost  of  the  work  to 
the  contractor  is  to  be  paid  by  the  principal,  plus,  (a)  a  fixed  sum 
of  money,  (6)  a  percentage  of  the  cost  of  the  work,  (c)  a  per- 
centage of  the  cost  of  the  work  but  with  a  fixed  limit,  (d)  a  per- 
centage of  the  difference  between  the  cost  of  the  work  and  some 
fixed  sum,  or  other  variations  of  this  principle.  Such  contracts 
have  the  advantage  that  the  principal  assumes  all  the  risk  in  con- 
struction and  therefore  pays  for  only  those  contingencies  which 
actually  arise.  Except  for  the  last  named  form,  they  have  the 
disadvantage  that  there  is  little  or  no  incentive  for  the  contractor 
to  keep  the  cost  of  the  work  down.  They  are  most  successful 
where  the  contractor  can  be  selected  by  the  principal,  but  where 

1  Business  Law  for  Engineers,  C.  Frank  Allen,  McGraw-Hill,  1917;  En- 
gineering Contracts  and  Specifications,  J.  B.  Johnson,  McGraw-Hill,  1904; 
Contracts  in  Engineering,  J.  I.  Tucker,  McGraw-Hill,  1910;  The  Law  Affect- 
ing Engineers,  W.  V.  Ball,  Archibald  Constable,  1909;  Law  and  Business 
of  Engineering  and  Contracting.  C.  E.  Fowler,  McGraw-Hill,  1909;  The 
Economics  of  Contracting,  D.  J.  Hauer,  E.  H.  Baumgartner,  1915;  The 
Elements  of  Specification  Writing,  R.  S.  Kirby,  John  Wiley  &  Son,  1913; 
Contracts,  Specifications  and  Engineering  Relations,  D.  W.  Mead,  McGraw- 
Hill,  1916;  Engineering  and  Architectural  Jurisprudence,  J.  C.  Wait,  John 
Wiley,  1912, 


THE  AGREEMENT  213 

it  is  necessary  to  let  contracts  to  the  lowest  bidder,  the  "  cost- 
plus  "  contract  is  not  easily  managed.  In  most  states  a  munici- 
pality cannot  make  a  cost-plus  contract. 

A  unit-price  contract  is  one  in  which  the  amount  to  be  paid  is 
fixed  in  proportion  to  the  amount  of  work  done  or  materials  sup- 
plied. This  type  of  contract  is  the  most  suitable  for  sewer  con- 
struction for  a  municipality  where  the  contract  must  be  let  to  the 
lowest  bidder.  The  contractor  is  protected  in  the  event  of  many 
unforeseen  emergencies  and  the  principal  is  protected  against  a 
raise  in  bids  to  cover  such  emergencies  and  against  increase  in  the 
cost  of  the  work  in  order  to  increase  the  profits  under  a  "  cost- 
plus  "  contract. 

It  is  sometimes  desirable  for  the  principal  to  furnish  a  portion 
of  the  materials,  the  bidders  being  notified  beforehand  that  this 
material  will  be  furnished.  In  this  manner  the  quality  of  material 
is  assured,  contractors  with  the  necessary  skill  but  small  capital 
may  be  attracted  to  bid,  and  uncertainties  in  the  procuring  of 
materials  is  eliminated. 

109.  The  Agreement. — A  contract  is  an  agreement  between 
two  or  more  interested  parties  to  do  a  certain  thing.  A  contract 
for  the  construction  of  a  sewer  is  an  agreement  between  a  muni- 
cipality or  individual  desiring  sewerage  facilities  and  a  company  or 
individual  engaged  in  the  construction  of  sewers.  The  latter 
promises  to  construct  a  sewer  in  return  for  which  the  former 
promises  to  pay  a  certain  amount  of  money. 

The  various  portions  of  the  agreement  which  are  bound 
together  as  the  complete  contract  are:  I.  The  Advertisement, 
II.  Information  and  Instructions  for  Bidders,  III.  Proposal, 
IV.  General  Specifications,  V.  Technical  Specifications,  VI. 
Special  Specifications,  VII.  Contract,  VIII.  Bond,  and  IX. 
Contract  Drawings.  These  should  be  fastened  together  in  pamph- 
let form  and  constitute  the  complete  instrument  called  the  con- 
tract. No  binding  contract  and  specifications  can  be  drawn  upon 
logical  deductions  alone  as  legal  precedent  and  tried  methods 
must  be  followed  to  insure  success.  To  draw  up  an  original  con- 
tract requires  the  combined  knowledge  of  an  engineer  and  a 
lawyer.  The  engineer  of  to-day  writes  his  specifications  by  copy- 
ing copiously  from  specifications  used  on  work  which  has  been 
completed  successfully.  In  order  that  selections  may  be  made 
with  judgment  and  discrimination  some  examples  have 


214  CONTRACTS  AND  SPECIFICATIONS 

been  selected   from  existing   published   specifications  and  con- 
tracts. 

110.  The  Advertisement.— This  should  contain:  (1)  A  head- 
ing indicating  the  type  of  work,  (2)  A  statement  as  to  when, 
where  and  how  bids  will  be  received  and  opened,  (3)  A  brief 
description  of  the  character  and  amount  of  work  to  be  done,  (4) 
The  method  of  payment,  (5)  The  conditions  under  which  further 
information  can  be  obtained,  (6)  A  statement  as  to  the  amount 
of  money  which  must  be  deposited  with  the  bid,  and  (7)  Any 
other  pertinent  facts  concerning  the  work.1  An  example  of  an 
advertisement  follows ; 

Sewer  Construction 
Construction  Turkey  Creek  Sewer 

Kansas  City,  Missouri. 

Bids  for  the  construction  of  the  Turkey  Creek  Sewer,  two  sewage 
pumping  stations  to  be  used  in  connection  therewith,  and  certain 
laterals  and  extensions  of  existing  sewers  thereto,  for  Kansas  City, 
Missouri,  will  be  received  up  to  2  p.  m.  August  19,  1919,  at  the  office 
of  the  Board  of  Public  Works,  City  Hall,  Kansas  City,  Missouri. 

The  main  sewer  will  be  about  one  and  one-fifth  miles  long,  and  the 
laterals  and  extensions  about  three  and  one-half  miles:  the  main 
sewer  will  be  constructed  of  reinforced  concrete,  the  laterals  and 
extensions  will  consist  of  concrete,  segment  blocks,  and  clay  pipe. 

This  work  is  estimated  to  cost  from  $1,500,000  to  $1,750,000. 
Payment  for  the  work  will  be  made  in  four  year  special  tax  bills, 
bearing  7  per  cent  interest,  payable  one-fourth  each  year.  Time 
600  working  days,  barring  strikes,,  bad  weather,  etc. 

Bidders  are  required  to  deposit  $15,000  in  cash  or  a  certified  check 
with  bid,  to  insure  signing  of  contract  when  let.  Same  to  be  returned 
on  execution  of  the  contract  or  rejection  of  bid. 

Complete  plans  and  specifications  for  the  work  may  be  had  and 
all  information  obtained  by  seeing  or  writing  to  A.  D.  Ludlow, 
Engineer  of  Sewers,  City  Hall,  Kansas  City,  Missouri.  Twenty-five 
($25.00)  Dollars  will  be  required  to  be  deposited  for  a  set  of  the  plans, 
but  $20.00  thereof  will  be  refunded  upon  return  of  the  plans  in  good 
condition. 

BOARD  OF  PUBLIC  WORKS, 

Kansas  City,  Missouri, 

by  F.  E.  McCabe,  Secretary. 

There  are  usually  legal  restrictions  which  require  that  the 
advertisement  be  inserted  a  certain  number  of  times  in  specified 
newspapers  or  other  advertising  mediums  before  the  opening  of 
bids.  If  the  contract  is  of  sufficient  size  to  attract  outside  con- 

*  See  article  by  E.  W.  Bush  in  Eng.  News-Record,  Vol.  85,  1920,  p.  122. 


•    INFORMATION  AND  INSTRUCTIONS  FOR  BIDDERS      215 

tractors,  the  advertisement  should  be  inserted  in  engineering  and 
contracting  journals  of  wide  circulation.  Although  the  adver- 
tisement appears  separately  from  the  other  portions  of  the  con- 
tract, a  copy  is  usually  bound  in  as  the  first  page  of  the  pamphlet 
containing  the  contract  and  specifications  and  is  made  an  integral 
part  thereof. 

111.  Information  and  Instructions  for  Bidders. — This  is  some- 
what on  the  order  of  an  introduction  to  the  pamphlet  in  which  the 
specifications,  contract,  and  contract  drawings  are  published. 
As  examples  of  the  type  of  information  and  instructions  given  to 
prospective  bidders  the  abstracts  below  have  been  taken  from  the 
"  Contract,  Specifications,  Bond,  and  Proposal  for  the  North 
Shore  Sanitary  Intercepting  Sewer  "  by  the  Sanitary  District  of 
Chicago.  The  information  and  instructions  to  bidders  can  be 
divided  into  the  following  sections:  1st.  Examination  of  Site, 
2nd.  Character  and  Quantity  of  Work,  3rd.  Qualification  for 
Bidding,  4th.  Instructions  for  Making  out  Proposal,  5th.  Certified 
Check,  and  6th.  Rejection  of  Bids, 

REQUIREMENTS   FOR   BIDDING   AND   INSTRUCTIONS  TO   BIDDERS 

Bidders  are  required  to  submit  their  bids  upon  the  following 
express  conditions: 

Bidders  must  carefully  examine  the  entire  sites  of  the 
work  and  the  adjacent  premises,  and  the  various  means 
of  approach  to  the  sites,  and  shall  make  all  necessary 
investigations  to  inform  themselves  thoroughly  as  to  the 
facilities  for  delivering  and  handling  materials  at  the  sites 
and  to  inform  themselves  thoroughly  as  to  all  the  difficulties 
that  may  be  involved  in  the  complete  execution  of  all  work 
under  the  attached  contract  in  accordance  with  the  speci- 
fications hereto  attached. 

Bidders  are  also  required  to  examine  all  maps,  plans, 
and  data  mentioned  in  the  specifications,  contract  or  pro- 
posal as  being  on  file  in  the  office  of  the  Chief  Engineer, 
for  examination  by  bidders.  No  plea  of  ignorance  of 
conditions  that  exist  or  that  may  hereafter  exist  or  of 
conditions  or  difficulties  that  may  be  encountered  in  the 
execution  of  the  work  under  this  contract,  as  the  result 
of  a  failure  to  make  the  necessary  examinations  and  investi- 
gations, will  be  accepted  as  an  excuse  for  any  failure  or 
omission  on  the  part  of  the  Contractor  to  fulfill  in  every 
detail  all  of  the  requirements  of  said  contract,  specifica- 


216  CONTRACTS  AND  SPECIFICATIONS 

tions  and  plans,  or  will  be  accepted  as  a  basis  for  any 
claims  whatsoever  for  extra  compensation.  Upon  applica- 
tion all  information  in  the  possession  of  the  Chief  Engineer 
will  be  shown  to  bidders,  but  the  correctness  of  such 
information  will  not  be  guaranteed  by  the  Sanitary  District. 
The  following  schedule  of  quantities,  although  stated 
with  as  much  accuracy  as  :s  possible  in  advance,  is  approxi- 
mate only,  and  is  assumed  solely  for  the  purpose  of  com- 
paring bids. 

Then  follows  an  itemized  schedule  of  the  quantity  of  work  to  be 
done  after  which  comes  the  following: 

Bidders  must  determine  for  themselves  the  quantities 
of  work  that  will  be  required,  by  such  means  as  they  may 
prefer,  and  shall  assume  all  risks  as  to  variations  in  the 
quantities  of  the  different  classes  of  work  actually  furnished 
under  the  contract.  Bidders  shall  not  at  any  time  after 
the  submission  of  this  proposal,  dispute  or  complain  of 
the  aforesaid  schedules  of  quantities  or  assert  that  there 
was  any  misunderstanding  in  regard  to  the  amount  or  the 
character  of  the  work  to  be  done,  and  shall  not  make  any 
claims  for  damages  or  for  loss  of  profits  because  of  a  dif- 
ference between  the  quantities  of  the  various  classes  of 
work  assumed  for  comparison  of  bids  and  the  quantities 
of  work  actually  performed. 

Proposals  that  contain  any  omissions,  erasures,  or 
alterations,  conditions  or  items  not  called  for  in  the  con- 
tract and  plans  attached  hereto,  or  that  contain  irregu- 
larities of  any  kind,  will  be  rejected  as  informal. 

Bids  manifestly  unbalanced  will  not  be  considered  in 
awarding  the  contract.1 

No  bid  will  be  accepted  unless  the  party  making  it 
shall  furn'sh  evidence  satisfactory  to  the  Board  of  Trustees 
of  the  Sanitary  District  of  Chicago  of  his  experience  and 
familiarity  with  work  of  the  character  specified  and  of 
his  financial  ability  to  successfully  and  properly  prosecute 
the  proposed  work  to  completion  within  the  specified  time. 

Each  bid  shall  be  accompanied  by  a  certified  check, 
or  cash,  to  the  amount  of  ten  (10)  per  cent  of  the  total 
amount  of  said  bid  figured  on  the  quantities  given  here- 

1An  unbalanced  proposal  is  one  in  which  the  bids  on  some  of  the  items 
are  obviously  low  and  on  other  items  are  obviously  or  suspiciously  high. 
The  purpose  of  submitting  unbalanced  bids  is  to  keep  secret  the  true  or 
supposed  cost  of  the  work  to  the  contractor  or  to  obtain  more  money  by  bid- 
ding high  on  those  items  which  are  believed  to  have  been  underestimated 
by  the  Engineer.  A  low  bid  is  made  on  other  items  in  order  to  keep  down 
the  total  amount  of  the  bid. 


PROPOSAL  217 

with,  the  lowest  alternative  total  being  allowed.  Said 
amounts  deposited  with  bids,  shall  be  held  until  all  of  the 
bids  have  been  canvassed  and  the  contract  awarded  and 
signed.  The  return  of  said  check  or  cash  to  the  bidder  to 
whom  the  contract  for  said  work  is  awarded  will  be  con- 
ditioned upon  his  appearing  and  executing  a  contract  for 
the  work  so  awarded  and  giving  bond  satisfactory  to  said 
Board  of  Trustees,  for  the  fulfillment  of  each  contract  in 
the  amount  of  fifty  (50)  per  cent  of  the  amount  of  each 
contract. 

The  said  Board  of  Trustees  reserves  the  right  to  re- 
ject any  or  all  bids. 

Accompanying  the  contract  form  are  plans  which, 
together  with  the  specifications,  show  the  work  on  which 
said  tenders  are  to  be  made. 

The  proposal  must  not  be  detached  herefrom  or  from 
the  contract  by  any  bidder  when  submitting  a  bid. 

112.  Proposal. — The  proposal  is  a  blank  printed  form  on 
which  the  bidder  is  required  to  enter  the  prices  for  which  he 
proposes  to  do  the  work.  The  proposal  blank  is  necessary  in 
order  that  the  bids  may  be  sufficiently  uniform  for  proper  com- 
parison. Sewers  are  often  paid  for,  particularly  for  small  sizes, 
per  foot  of  completed  sewer  as  measured  along  the  center  line  of 
the  pipe  parallel  to  the  surface  of  the  ground  with  the  exterior 
length  of  manholes  and  other  structures  deducted.  Sometimes, 
under  other  conditions,  a  different  rate  is  allowed  for  each  addi- 
tional two  feet  of  depth  of  sewer,  and  special  structures,  such  as 
manholes,  catch-basins,  flush-tanks,  etc.,  are  paid  for  at  a  unit 
price  according  to  the  depth.  Water  connections  to  flush-tanks 
are  paid  for  per  foot  of  length  of  service  pipe  laid.  In  especially 
large  or  difficult  work,  materials  are  paid  for  at  a  unit  price,  for 
example,  per  cubic  yard  of  excavation,  per  cubic  yard  of  concrete, 
per  thousand  feet  board  measure  of  lumber,  etc. 

The  following  example  is  taken  from  the  contract  for  the 
North  Shore  Intercepting  Sewer  previously  quoted,  to  indicate 
the  type  of  Proposal  used; 


218  CONTRACTS  AND  SPECIFICATIONS 

PROPOSAL 

FOR  THE  CONSTRUCTION  OF  THE  NORTH  SHORE 
INTERCEPTING  SEWER 

To  the  Honorable,  the  President  and  the  Board  of  Trustees 
of  the  Sanitary  District  of  Chicago: 

Gentlemen: — The    undersigned    hereby    certi 

that ha examined  the  specifications  and  form 

of  contract  and  the  accompanying  plans  for  the  construc- 
tion of  the  North  Shore  Intercepting  sewer,  and  ha 

also  examined  the  premises  at  and  adjacent  to  the  sites 
of  the  proposed  work,  as  herein  described,  and  the  means 
of  approach  to  the  said  sites. 

The  undersigned  ha also  examined  the  fore- 
going "  Requirements  for  Bidding  and  Instructions  to 
Bidders  "  and  propose.  ...  to  do  all  the  work  called  for  in 
said  specifications  and  contract,  and  shown  on  said  plans, 
and  to  furnish  all  materials,  tools,  labor  and  all  appliances 
and  appurtenances  necessary  to  the  full  completion  of 
said  work  at  the  rates  and  prices  for  said  work  as  follows, 
to-wit: 

(la)  For  six  (6)  by  nine  (9)  foot  concrete  sewer,  com- 
plete in  place,  as  specified,  the  sum  of 

Dollars   and 

cents  ($ )  per  linear  foot. 

(6a)  For  manholes,  concrete,  complete  in  place,  as 
specified  the  sum  of 

! Dollars 

and cents  ($ )  each 

The  following  plans  showing  the  work  to  be  performed 
in  accordance  with  the  attached  specifications,  have  been 
examined  by  the  undersigned  in  preparing  the  foregoing 
proposal,  to-wit: 


In   accordance   with   the   requirements   set   forth   in   the 
attached  Information  and  Instructions  for  Bidders,  there 

is  deposited  herewith  the  sum  of 

Dollars  and 

cents  ($ )  which 

under  the  terms  therein  mentioned  entitle . . 


to  bid  on  said  work,  the  said  sum  to  be  refunded  to , 


upon  the  faithful  performance  of  all  conditions  set  forth 
in  the  Information  and  Instructions  for  Bidders. 

Name 

Address. . 


GENERAL  SPECIFICATIONS  219 

Blanks  are  provided  for  each  item.  No  place  is  left  at  the 
end  for  a  summary.  The  proposal  ends  with  an  acknowledgment 
that  the  contract  has  been  examined  completely  and  all  prelimi- 
nary directions  therein  have  been  complied  with.  A  blank  is 
prepared  for  inserting  the  amount  of  the  required  certified  check, 
and  finally  for  the  signature  of  the  bidders. 

113.  General  Specifications. — The  specifications,  both  general 
and  technical,  are  occasionally  incorporated  in  the  contract  form, 
but  more  frequently  they  are  printed  separately  and  are  bound  in 
the  pamphlet  preceding  the  contract.  The  general  specifications 
relate  to  the  conditions  under  which  all  work  must  be  performed 
and  are  as  applicable  to  the  construction  of  a  pumping  station  as 
to  the  smallest  lateral,  unless  otherwise  specified.  It  is  not  pos- 
sible to  include  a  complete  set  of  General  Specifications  in  the 
limited  space  of  this  text,  but  the  more  important  specifications 
will  be  emphasized  by  examples  taken  from  specifications  in  use.1 

The  subjects  covered  in  General  Specifications  are: 

(1)  Definitions  of  doubtful  terms. 

(2)  The  Engineer  to  settle  disputes. 

(3)  Duties  of  the  Engineer. 

(4)  Duties  of  the  Contractor. 

(5)  Hours  and  days  of  work. 

(6)  No  work  to  be  done  in  the  absence  of  an  inspector. 

(7)  Contractor  to  be  represented  at  all  times. 

(8)  Time  of  commencing  and  completing  the  work. 

(9)  Liquidated  damages  for  delay  in  completion. 

(10)  The  City  may  change  the  plans. 

(11)  The  City  may  increase  the  amount  of  the  work. 

(12)  Inspection  and  its  conduct. 

(13)  The  Contractor  to  be  acquainted  with  laws  relat- 

ing to  the  work. 

(14)  Contractor  responsible  for  damages  to  persons  or 

property. 

(15)  City  to  be  protected  against  patent  claims. 

(16)  Abandonment  of  contract  and  its  remedy. 

(17)  Estimates  of  work  done  and  moneys  due. 

(18)  Payments  for  extra  work. 

(19)  Character  of  workmen  to  be  employed. 

(20)  City  may  reserve  a  sum  for  repairs  during  stipu- 

lated term  after  completion. 

1  Taken  mainly  from  specifications  of  the  Sanitary  District  of  Chicago 
and  the  Baltimore  Sewerage  Commission,  with  miscellaneous  selections 
from  other  sources. 


220  CONTRACTS  AND  SPECIFICATIONS 

(21)  City  may  use  money  due  Contractor  to  pay  claims 

for  labor  or  materials  used  on  the  work  and  not 
paid  for  by  the  Contractor. 

(22)  The  Contractor  shall  have  no  claim  for  damages 

on  account  of  delay  or  unforeseen  difficulties. 

(23)  The  Contractor  may  not  assign  nor  sublet  the 

contract  without  the  City's  consent. 

(24)  Cleaning  up  after  completion. 

(25)  The  Contractor's  relations  to  other  contractors. 

(26)  The  portions  composing  the  contract. 

The  following  examples  cover  the  subjects  named  in  the  pre- 
ceding titles: 

1.  Definitions.     The    word    Engineer    whenever    not 
qualified  shall  mean  the  Chief  Engineer  of  the  Commission, 
acting  either  directly  or  through  his  properly  authorized 
agents,  such  agents  acting  severally  within  the  scope  of 
the  particular  duties  entrusted  to  them. 

This  article  may  include  words  that  may  be  in  dispute  or  ambigu- 
ous such  as:  Board  of  Trustees,  Elevation,  City,  Contractor, 
Rock,  Earth,  etc.,  etc. 

2.  Disputes.     To  prevent  disputes  and  litigations,  the 
Engineer  shall  in  all  cases  determine  the  amount,  quality, 
and  acceptability  of  the  work  which  is  to  be  paid  for  under 
the  contract;   shall  decide  all  questions  in  relation  to  said 
work  and  the  performance  thereof,  and  shall  in  all  cases 
decide  every  question  which  may  arise  relative  to  the  ful- 
fillment of  the  contract  on  the  part  of  the  Contractor. 
His   determination,   decision  and  estimate  shall  be  final 
and  conclusive,  and  in  case  any  question  shall  arise  between 
the   parties   touching   the   contract,    such   determination, 
decision,  and  estimate  shall  be  a  condition  precedent  to 
the  right  of  the  Contractor  to  receive  any  moneys  under 
the  contract. 

3.  Duties  of  the  Engineer.     The  Engineer  shall  make 
all  necessary  explanations  as  to  the  meaning  and  intentions 
of  the  specifications  and  shall  give  all  orders  and  directions, 
either  contemplated  therein  or  thereby,  or  in  every  case 
in  which  a  difficulty  or  unforeseen  condition  shall  arise  in 
the  performance  of  the  work.     Should  there  be  any  dis- 
crepancies in  or  between,  or  should  any  misunderstanding 
arise  as  to  the  import  of  anything  contained  in  the  plans 
and  specifications,  the  decision  of  the  Engineer  shall  be 
final  and  binding.     Any  errors  or  omissions  in  plans  and  spe- 
cifications may  be  corrected  by  the  Engineer,  when  such 
corrections  are  necessary  for  the  proper  fulfillment  of  their 
intentions  as  construed  by  him. 


GENERAL  SPECIFICATIONS  221 

4.  Duties   of   the   Contractor.     The   Contractor   shall 
do  all  the  work  and  furnish  all  the  labor,  materials,  tools 
and  appliances  necessary  or  proper  for  performing  and  com- 
pleting the  work  required  by  the  contract,  in  the  manner 
called  for  by  the  specifications,  and  within  the  contract 
time.     He  shall  complete  the  entire  work  at  the  prices 
agreed  upon  and  fixed  therefor  to  the  satisfaction  of  the 
Commission  and  its   Chief  Engineer  and  in  accordance 
with  the  specifications,  the  drawings,  and  such  detailed 
drawings  as  may  be  furnished  from  time  to  time,  together 
with  such  extra  work  as  may  be  required  for  the  perform- 
ance of  which  written  orders  may  be  given  and  received 
as  hereinafter  provided. 

The  Contractor  shall  place  sufficient  lights  on  or  near 
the  work  and  keep  them  burning  from  twilight  to  sunrise; 
shall  erect  suitable  railings,  fences  or  other  protections 
about  all  open  trenches,  and  provide  all  watchmen  on  the 
work,  by  day  or  night,  that  may  be  necessary  for  the  public 
safety.  The  Contractor  shall,  upon  notice  from  the  Engi- 
neer that  he  has  not  satisfactorily  complied  with  the  fore- 
going requirements,  immediately  take  such  methods  and 
provide  such  means  and  labor  to  comply  therewith  as  the 
Engineer  may  direct,  but  the  Contractor  shall  not  be 
relieved  of  this  obligation  under  the  contract  by  any  such 
notice  or  directions  given  by  the  Engineer,  or  by  neglect, 
failure,  or  refusal  on  the  part  of  the  Engineer  to  give  such 
notice  and  directions. 

The  Contractor  shall  furnish  such  stakes  and  the  neces- 
sary labor  for  driving  them  as  may  be  required  by  the 
Engineer.  He  shall  maintain  the  stakes  when  set,  with 
reasonable  diligence,  and  stakes  misplaced  due  to  the  careless- 
ness of  the  Contractor  or  his  workmen  shall  be  reset  under 
the  direction  of  the  Engineer,  at  the  Contractor's  expense. 

5.  Night,    Sunday,  and    Holiday    Work:1    No    night, 
Sunday,  nor  holiday  work  requiring  the  presence  of  an 
engineer  or  inspector  will  be  permitted  except  in  case  of 
emergency,  and  then  only  to  such  an  extent  as  is  absolutely 
necessary  and  with  the  written  permission  of  the  Engineer; 
provided  that  this  clause  shall  not  operate  in  the  case  of 
a  gang  organized  for  regular  and  continuous  night,  Sunday, 
or  holiday  work. 

6.  Absence  of  Engineer  or  Inspector.     Any  work  done 
without  lines,  levels,  and  instructions  having  been  given 
by  the  Engineer  or  without  the  supervision  of  an  assistant 

1  Restrictions  are  placed  on  work  done  outside  of  ordinary  working  hours 
in  order  that  the  Contractor  may  not  perform  work  in  the  absence  of  an 
engineer  or  inspector. 


222  CONTRACTS  AND  SPECIFICATIONS 

or  inspector,  will  not  be  estimated  or  paid  for  except  when 
such  work  is  authorized  by  the  Engineer  in  writing.  Work 
so  done  may  be  ordered  removed  and  replaced  at  the 
Contractor's  sole  cost  and  expense. 

7.  Absence  of  Contractor.     During  the  absence  of  the 
Contractor  he  shall  at  all  times  have  a  duly  authorized 
representative  on  the  work.     The  Contractor  shall  give 
written  notice  to  the  Commission  of  the  name  and  address 
of  said  representative  and  shall  state  where  and  how  such 
representative  can  be  reached,  at  any  and  all  hours,  whether 
by  day  or  night. 

Whenever  the  Contractor  or  his  representative  is  not 
present  at  any  place  on  the  work  where  it  may  be  necessary 
to  give  orders  or  directions,  such  orders  or  directions  will 
be  given  by  the  Engineer  and  they  shall  be  received  and 
promptly  obeyed  by  the  superintendent  or  foreman  who 
may  have  immediate  charge  of  the  particular  work  in  rela- 
tion to  which  the  order  may  be  given. 

8.  Commencing    Work.     The    Contractor    agrees    to 

begin  the  work  covered  by  this  contract  within  

days  of  the  execution  of  the  contract  and  to  prosecute  the 
same  with  all  due  diligence  and  to  entirely  complete  the 
work  within days. 

It  is  understood  and  agreed  that  time  is  of  the  essence 
of  this  contract,  and  that  a  failure  on  the  part  of  the  Con- 
tractor to  complete  the  work  herein  specified  within  the 
.  time  specified  will  result  in  great  loss  and  damage  to  said 
Sanitary  District  and  that  on  account  of  the  peculiar 
nature  of  such  loss  it  is  difficult,  if  not  impossible,  to  accu- 
rately ascertain  and  definitely  determine  the  amount 
thereof.  . 

9.  Liquidated   Damages.     It   is   therefore   covenanted 
and  agreed  that  in  case  the  said  Contractor  shall  fail  or 
neglect  to  complete  the  work  herein  specified  on  or  before 
the  date  hereinbefore  fixed  for  completion,  the  said  Con- 
tractor shall  and  will  pay  the  said  Sanitary  District  the 
sum  of Dollars  for  each  and  every  day  the  Con- 
tractor shall  be  in  default  in  the  time  of  completion  of 
this  contract. 

Said  sum  of Dollars  per  day  is  hereby  agreed 

upon,  fixed  and  determined  by  the  parties  hereto  as  the 
liquidated  damages  which  said  Sanitary  District  will 
suffer  by  reason  of  such  defaults,  and  not  by  way  of  a 
penalty. 

10.  Changes  in  Plans.     The  Board  reserves  the  right 
to  change  the  alignment,  grade,  form,  length,  dimensions 
or  materials  of  the  sewers  or  any  of  their  appurtenances, 
whenever   any   condition   or   obstructions   are   met    that 


GENERAL  SPECIFICATIONS  223 

render  such  changes  desirable  or  necessary.  In  case  the 
alterations  thus  ordered  make  the  work  less  expensive  to 
the  Contractor  a  proper  deduction  shall  be  made  from 
the  contract  prices  and  the  Contractor  shall  have  no  claim 
on  this  account  for  damages  or  for  anticipated  profits  on 
the  work  that  may  be  dispensed  with.  In  case  such 
alterations  make  the  work  more  expensive,  a  proper  addi- 
tion shall  be  made  to  the  contract  prices.  Any  deduction 
or  addition  as  aforesaid  shall  be  determined  and  fixed  by 
the  Engineer. 

11.  Extensions  and  Additions.     In  the  event  that  any 
material  alterations  or  additions  are  made  as  herein  speci- 
fied  which  in  the   opinion   of  the   Engineer  will  require 
additional  time  for  execution  of  all  the  work  under  this 
contract,  then,  in  that  case  the  time  of  completion  of  the 
work  shall  be  extended  by  such  a  period  or  periods  of  time 
as  may  be  fixed  by  said  Engineer  and  his  decision  shall  be 
final  and  binding  upon  both  parties  hereto,  provided  that 
in  such  case  the  Contractor,  within  four  (4)  days  after 
being  notified  in  writing  of  such  alterations  and  additions, 
shall  request   in  writing    an   extension  of  time,   but   the 
provisions  of  this  paragraph  shall  not  otherwise  alter  the 
provisions  of  this   contract  with   reference   to   liquidated 
damages,  and  the  said  Contractor  shall  not  be  entitled  to 
any   damages   or   compensation   from   the   said   Sanitary 
District  on  account  of  such  additional  time  required  for 
the  execution  of  the  work. 

12.  Inspection.     All  materials  of  whatsoever  kind  to 
be  used  in  the  work  shall  be  subject  to  the  inspection  and 
approval  of  the  Engineer  and  shall  be  subject  to  constant 
inspection  before  acceptance.     Any  imperfect  work  that 
may  be  discovered  before  its  final  acceptance  shall  be  cor- 
rected immediately,  and  any  unsatisfactory  materials  used 
in  the  work  or  delivered  at  the  site  shall  be  rejected  and 
removed  on  the  requirement  of  the  Engineer.     The  inspec- 
tion of  any  work  shall  not  relieve  the  Contractor  of  any 
of  his  obligations  to  perform  proper  and  satisfactory  work 
as  herein  specified,  and  all  work  which,  during  the  progress 
and  before  the  final  acceptance,  may  become  damaged 
from  any  cause,  shall  be  removed  and  replaced  by  good 
and  satisfactory  work  without  extra  charge  therefor.     The 
Engineer  and  his  assistants  shall  have  at  all  times  free 
access  to  every  part  of  the  work  and  to  all  points  where 
material  to  be  used  in  the  work  is  manufactured,  procured 
or  stored  and  shall  be  allowed  to  examine  any  material 
furnished  for  use  in  the  work  under  this  contract. 

All  inspection  of  any  and  all  material  furnished  for  use 
in  work  to  be  performed  under  this  contract  shall  be  made 


224  CONTRACTS  AND  SPECIFICATIONS 

at  the  site  of  the  work  after  the  delivery  of  the  material, 
provided,  that,  if  requested  by  the  Contractor  the  Engi- 
neer may  at  his  option  perform,  or  have  performed,  inspec- 
tion of  materials  at  points  other  than  the  site  of  the  work. 
In  any  such  case  the  Contractor  shall  pay  the  Sanitary  Dis- 
trict the  extra  cost  of  such  inspection,  including  the  neces- 
sary expenses  of  the  inspector  for  the  extra  time  expended 
in  performing  any  such  inspection  at  said  other  points. 

13.  Legal  Requirements.     The  Contractor  shall  keep 
hinself  fully  informed  of  all  existing  and  future  national 
and  state  laws  and  local  ordinances  and  regulations  in 
any  manner  affecting  those  engaged  or  employed  in  the 
work,  or  the  materials  used  in  the  work,  or  of  all  such 
orders  and  degrees  of  bodies  or  tribunals  having  any  juris- 
diction or  authority  over  the  same,  and  shall  protect  and 
indemnify  the  party  of  the  first  part  against  any  claim 
or  liability  arising  from  or  based  on  the  violation  of  such 
law,  ordinance,  regulation,  order   or   decree,  whether   by 
himself  or  his  employees. 

14.  Damages.     If  any  damage  shah  be  done  by  the 
Contractor  or  by  any  person  or  persons  in  his  employ  to 
the  owner  or  occupants  of  any  land  or  to  any  real  or  per- 
sonal property  adjoining,  or  in  the  vicinity  of  the  work 
herein  contracted  to  be  done  or  to  the  property  of  a  neigh- 
boring contractor  the  Engineer  shall  have  the  right  to  esti- 
mate the  amount  of  said  damage  and  to  cause  the  Sanitary 
District  to  pay  the  same  to  the  said  owner,  occupant,  or 
contractor,  and  the  amount  so  paid  shall  be  deducted 
from  the  money  due  said  Contractor  under  this  contract. 
Said  Contractor  covenants  and  agrees  to  pay  all  damages 
for  any  personal  injury  sustained  by  any  person  growing 
out  of  any  act  or  doing  of  himself  or  his  employees  that 
is  in  the  nature  of  a  legal  liability,  and  he  hereby  agrees 
to   indemnify   and   save   the   Sanitary   District   harmless 
against  all  suits  or  actions  of  every  name  and  description 
brought  against  said  Sanitary  District,  for  or  on  account 
of  any  such  injuries,  or  such  damages  received  or  sustained 
by  any  person  or  persons;  and  the  said  Contractor  further 
agrees  that  so  much  of  the  money  due  to  him  under  this 
contract,  as  shall  be  considered  necessary  by  the  Board  of 
Trustees  of  said  Sanitary  District,  may  be  retained  by  the 
Sanitary  District  until  such  suit  or  claim  for  damages 
shall  have  been  settled,  and  evidence  to  that  effect  shall 
have  been  furnished  to  the  satisfaction  of  said  Board  of 
Trustees. 

15.  Patents.    It  is  further  agreed  that  the  Contractor 
shall  indemnify,   keep  and   save  harmless  said   Sanitary 
District  from  all  liabilities,  judgments,  costs,  damages  and 


GENERAL  SPECIFICATIONS  225 

expenses  which  may  in  any  wise  come  against  said  Sanitary- 
District,  or  which  may  be  the  result  of  an  infringement 
of  any  patent  by  reason  of  the  use  of  any  materials,  machin- 
ery, devices,  apparatus,  or  process  furnished  or  used  in 
the  performance  of  this  contract,  or  by  reason  of  the  use 
of  designs  furnished  by  the  Contractor  and  accepted  by 
the  Sanitary  District,  and  in  the  event  of  any  claim  or 
suit  or  action  at  law  or  in  equity  of  any  kind  whatsoever 
being  made  or  brought  against  said  Sanitary  District,  then 
the  Sanitary  District  shall  have  the  right  to  retain  a  suffi- 
cient amount  of  money  in  the  same  manner  and  upon  the 
conditions  as  hereinafter  specified. 

16.  Abandonment  of  Contract.  If  the  work  to  be  done 
under  the  contract  shall  be  abandoned  by  the  Contractor, 
or  if  at  any  time  the  Engineer  shall  be  of  the  opinion,  and 
shall  so  certify,  in  writing,  to  the  Commission,  that  the 
performance  of  the  contract  is  unnecessarily  or  unreason- 
ably delayed,  or  that  the  Contractor  is  willfully  violating 
any  of  the  conditions  of  the  specifications,  or  is  executing 
the  same  in  bad  faith,  or  not  in  accordance  with  the  terms 
thereof,  or  if  the  work  be  not  fully  completed  within  the 
time  named  in  the  contract  for  its  completion,  the  Com- 
mission may  notify  the  Contractor  to  discontinue  all 
work  thereunder,  or  any  part  thereof,  by  a  written  notice 
served  upon  the  Contractor,  as  herein  provided;  and 
thereupon  the  Contractor  shall  discontinue  the  work,  or 
such  part  thereof,  and  the  Commission  shall  thereupon 
have  the  power  to  contract  for  the  completion  of  said  work 
in  the  manner  prescribed  by  law,  or  to  procure  and  furnish 
all  necessary  materials,  animals,  machinery,  tools  and 
appliances,  and  to  place  such  and  so  many  persons  as  it 
may  deem  advisable  to  work  at  and  complete  the  work 
described  in  the  specifications,  or  such  part  thereof,  and 
to  charge  the  entire  cost  and  expense  thereof  to  the  Con- 
tractor. And  for  such  completion  of  the  work  or  any  part 
thereof,  the  Commission  may  for  itself  or  its  contractors, 
take  possession  of  and  use  or  cause  to  be  used  any  or  all 
such  materials,  animals,  machinery,  tools  and  implements 
of  every  description  as  may  be  found  on  the  line  of  the  said 
work.  The  cost  and  expense  so  charged  shall  be  deducted 
from,  and  paid  by  the  City  out  of  such  moneys  as  may  be 
due  or  may  become  due  to  the  Contractor,  under  and  by 
virtue  of  the  contract.  In  case  such  expense  shall  exceed 
the  amount  which  would  have  been  payable  under  the  con- 
tract, if  the  same  had  been  completed  by  the  Contractor, 
he  shall  pay  the  amount  of  such  excess  to  the  City.  When 
any  particular  part  of  the  work  is  being  carried  on  by  the 
Commission,  by  contract  or  otherwise,  under  the  provisions 


226  CONTRACTS  AND  SPECIFICATIONS 

of  this  clause  of  the  contract,  the  Contractor  shall  continue 
the  remainder  of  the  work  in  conformity  with  the  terms 
of  his  contract,  and  in  such  manner  as  in  no  wise  to  hinder 
or  interfere  with  the  persons  or  workmen  employed  by  the 
Commission  by  contract  or  otherwise  as  above  provided, 
to  do  any  part  of  the  work  or  to  complete  the  same  under 
the  provisions  hereof. 

17.  Estimates.  The  Engineer  shall  from  time  to  time 
as  the  work. progresses,  on  or  about  the  last  day  of  each 
month,  make  in  writing  an  estimate,  such  as  he  shall  believe 
to  be  just  and  fair,  of  the  amount  and  value  of  the  work 
done  and  the  materials  incorporated  into  the  work  by  the 
Contractor  under  the  specifications,  provided  however  that 
no  such  estimate  shall  be  required  to  be  made  when,  in  the 
judgment  of  the  Engineer  the  total  value  of  the  work  done 
and  the  materials  incorporated  into  the  work  since  the  last 

preceding  estimate  is  less  than  dollars.  Such 

estimates  shall  not  be  required  to  be  made  by  strict  measure- 
ments, but  they  may  be  approximate  only. 

The  Contractor  shall  not  be  entitled  to  demand  from 
the  Commission  as  a  right,  a  detailed  statement  of  the 
measurements  or  quantities  entering  into  the  several  items 
of  the  monthly  estimates,  but  he  will  be  given  such  oppor- 
tunities and  facilities  to  verify  the  estimates  as  may  be 
deemed  reasonable  by  the  Commission. 

When  in  the  opinion  of  the  Engineer,  the  Contractor 
shall  have  completely  performed  the  contract  on  his  part, 
the  Engineer  shall  make  a  final  estimate,  based  on  actual 
measurements,  of  the  whole  amount  of  the  work  under 
and  according  to  the  terms  of  the  contract,  and  shall  certify 
to  the  Commission  in  writing,  the  amount  of  the  final 
estimate  at  the  completion  of  the  work.  After  the  com- 
pletion of  the  work  the  City  shall  pay  to  the  Contractor 
the  amount  remaining  after  deducting  from  the  total 
amount  or  value  of  the  work,  as  stated  in  the  final  estimate, 
all  such  sums  as  have  theretofore  been  paid  to  the  Con- 
tractor under  any  of  the  provisions  of  the  contract,  except 
such  sums  as  may  have  been  paid  for  extra  work,  and  also 
any  sum  or  all  sums  of  money  which  by  the  terms  thereof 
the  City  is  or  may  be  authorized  to  reserve  or  retain; 
provided  that  nothing  therein  contained  shall  affect  the 
right  of  the  City,  hereby  reserved,  to  reject  the  whole 
or  any  portion  of  the  aforesaid  work,  should  the  said 
cert'ficate  be  found  or  known  to  be  inconsistent  with  the 
terms  of  the  contract  or  otherwise  improperly  given.  All 
monthly  estimates  upon  which  partial  payments  have  been 
made,  being  merety  estimates,  shall  be  subject  to  cor- 
rection in  the  final  estimate,  which  final  estimate  may  be 


GENERAL  SPECIFICATIONS  227 

made  without  notice  thereof  to  the  Contractor,  or  of  the 
measurements  upon  which  it  is  based. 

18.  Extra  Work.     The  Contractor  shall  do  any  work 
not  herein  otherwise  provided  for,  when  and  as  ordered 
in  writing  by  the  Engineer  or  his  agents  specially  authorized 
thereto  in  writing,  and  shall  when  requested  by  the  Engi- 
neer so  to  do,  furnish  itemized  statements  of  the  cost  of 
the  work  ordered  and  give  the  Engineer  access  to  accounts, 
bills,  vouchers,   etc.  relating  thereto.     If  the  Contractor 
claims  compensation  for  extra  work  not  ordered  as  aforesaid, 
or  for  any  damages  sustained,  he  shall  within  one  week 
after  the  beginning  of  any  such  work  or  the  sustaining 
of  any  such  damage,  make  a  written  statement  of  the 
nature  of  the  work  performed  or  the  damage  sustained, 
to  the  Engineer,  and  shall,  on  or  before  the  fifteenth  day 
of  the  month  succeeding  that  in  which  any  such  extra 
work  shall  have  been  done  or  any  such  damage  shall  have 
been  sustained,  file  with  the  Engineer  an  itemized  statement 
of  the  details  and  amount  of  any  such  work  or  damage ;  and 
unless  such  statement  shall  be  made  as  so  required,  his  claim 
for  compensation  shall  be  forfeited  and  he  shall  not  be  en- 
titled to  payment  on  account  of  any  such  work  or  damage. 

For  all  such  extra  work  the  Contractor  shall  receive 
the  reasonable  cost  of  said  work,  plus  fifteen  (15)  per  cent 
of  said  cost. 

19.  Competent    Employees.      The    Contractor    shall 
employ  only  competent  skillful  men  to  do  the  work;   and 
whenever  the   Engineer  shall   notify  the   Contractor,   in 
writing,  that  any  man  employed  on  the  work  is,  in  his 
opinion  unsatisfactory,  such  man  shall  be  discharged  from 
the  work  and  shall  not  again  be  employed  on  it,  except 
with  the  consent  of  the  Engineer. 

20.  Money   Retained.     Upon   the    completion   of   the 
work  and  its  acceptance  by  the  City,  the  City  shall  reserve 
and  retain  five  (5)  per  cent  of  the  total  value  of  the  work 
done  under  the  contract  as  shown  by  the  final  estimate, 
over  and  above  any  and  all  other  reservations  which  the 
city  by  the  terms  thereof  is  entitled  or  required  to  retain 
and  shall  hold  the  said  five  (5)  per  cent  for  a  period  of  nine 
(9)  months  from  and  after  the   date  of   completion  and 
acceptance,   and  the  City  shall  be   authorized  to  apply 
such  part  of  said  five  (5)  per  cent  so  retained  to  any  and 
all  costs  of  repairs  and  renewals  as  may  become  necessary 
during  such  period  of  nine  (9)  months,  due. to  improper 
work  done  or  materials  furnished  by  the  Contractor,  if 
the  Contractor  shall  fail  to  make  such  repairs  or  renewals 
within  twenty-four  (24)  hours  after  receiving  notice  from 
the  City  so  to  do. 


228  CONTRACTS  AND  SPECIFICATIONS 

Upon  the  expiration  of  said  nine  (9)  months  from  and 
after  the  completion  and  acceptance  of  the  work,  the  City 
shall  pay  to  the  Contractor  the  said  five  (5)  per  cent  hereby 
retained,  less  such  sums  as  may  have  been  retained  here- 
under. 

21.  Unpaid    Claims    against    Contractor.     The    Con- 
tractor shall  furnish  the  City  with  satisfactory  evidence 
that  all  persons  who  have  done  work  or  furnished  materials 
under  the  contract,  and  have  given  written  notices  to  the 
City,  before  and  within  ten  (10)  days  after  the  final  com- 
pletion and  acceptance  of  the  whole  work  under  the  con- 
tract, that  any  balance  for  such  work  or  materials  is  due 
and  unpaid,  have  been  fully  paid  or  satisfactorily  secured. 
And  in  case  such  evidence  is  not  furnished  as  aforesaid, 
such  amount  as  may  be  necessary  to  meet  the  claims  of 
the  persons  aforesaid  shall  be  fully  discharged  or  such 
notices  withdrawn. 

22.  Delays  and  Difficulties.     The  Contractor  shall  not 
be  entitled  to  any  claims  for  damages  on  account  of  post- 
ponement or  delay  in  the  work  occasioned  by  forces  beyond 
the  control  of  the  City,  nor  for  postponement  or  delay  in 
the  work  where  ten  (10)  days  written  notice  has  been  given 
the  Contractor  of  such  postponement  or  delay,  nor  where 
unforeseen  difficulties  are  encountered  in  the  prosecution 
of  the  work.     In  the  event  of  a  postponement  or  delay 
ordered  in  writing  by  the  City  the  time  of  completion  of 
the  contract  shall  be  extended  a  number  of  days  equal  to 
the  number  of  days  that  the  work  has  been  postponed  or 
delayed. 

23.  Assignment  of  Contract.     The  Contractor  shall  not 
assign  by  power  of  attorney  or  otherwise,  nor  sublet  the 
work  or  any  part  thereof,  without  the  previous  written 
consent  of  the  party  of  the  first  part,  and  shall  not  either 
legally  nor  equitably  assign  any  of  the  moneys  payable 
under  this  agreement  or  his  claim  thereto  unless  by  and 
with  the  consent  of  the  party  of  the  first  part. 

24.  Cleaning  Up.     On  or  before  the  completion  of  the 
work,  the  Contractor  shall,  without  charge  therefor,  tear 
down  and  remove  all  buildings  and  other  structures  built 
by  him,  shall  remove  all  rubbish  of  all  kinds  from  any 
grounds  which  he  has  occupied,  and  shall  leave  the  line 
of  the  work  in  a  clean  and  neat  condition. 

25.  Access  to  Work  and  Other  Contractors.     The  Com- 
mission and  its  engineers,  agents  and  employees  may  at 
any  time  and  for  any  purpose  enter  upon  the  work  and  the 
premises  used  by  the  Contractor,  and  the  Contractor  shall 
provide  proper  and   safe  facilities  therefor.     Other  con- 
tractors of  the  Commission  may  also  when  so  authorized 


TECHNICAL  SPECIFICATIONS  229 

by  the  Engineer,  enter  upon  the  work  and  the  premises 
used  by  the  Contractor  for  all  the  purposes  which  may 
be  required  by  their  contracts.  Any  differences  or  con- 
flicts which  may  arise  between  this  Contractor  and  other 
contractors  of  the  Commission  in  regard  to  their  work 
shall  be  adjusted  and  determined  by  the  Engineer. 

26.  The  Contract.  It  is  understood  and  agreed  by 
the  City  and  the  Contractor  that  the  terms  of  this  contract 
are  embodied  and  included  in  the  Advertisement.  Informa- 
tion and  Instructions  to  Bidders,  Proposal,  Specifications 
of  every  nature,  the  Bond  and  the  contract  drawings 
hereto  attached. 

These  few  articles  have  been  given  as  examples  of  some  of  the 
essential  subjects  to  be  treated  in  general  specifications.  It  is  to 
be  understood  that  these  examples  do  not  represent  a  complete 
set  of  general  specifications  and  items  have  been  omitted  the 
absence  of  which  in  a  complete  contract  might  be  injurious  to 
the  successful  completion  of  the  work. 

114.  Technical  Specifications. — These  ordinarily  follow  the 
general  specifications  and  have  to  do  with  the  quality  of  materials, 
the  manner  of  putting  them  together,  and  the  method  of  doing 
the  work.  The  subject  headings  in  the  Technical  Specifications 
on  the  Baltimore  Sewerage  Commission  are: 

Excavation  Cement 

Tunneling  Mortar 

Rock  Excavation  Concrete 

Sheeting  Brick 

Sheet  Piling  Masonry 

Sheeting  and  Bracing  Reinforced  Concrete 

Piles  Vitrified  Pipe 

Blasting  Concrete  and  Brick  Sewers 

Pumping  and  Drainage  Vitrified  Pipe  Sewers  and  Drains 

Foundations  Manholes 

Refilling  Iron  Castings 

Repaving  House  Connections 

Underdrains  Obstructions 

Buildings  Fences 

Inlets  and  Catch-Basins  Flush-Tanks 

Each  of  these  subjects  is  treated  in  the  appropriate  section  of 
this  book. 

An  important  part  of  each  section  of  the  technical  specifica- 
tions is  the  clause  providing  for  the  method  of  payment  for  the 
work  specified.  This  is  usually  the  last  clause  in  the  section. 


230  CONTRACTS  AND  SPECIFICATIONS 

For  example,    the   last   clause   in   the   Baltimore   Specifications 
relating  to  Rock  Excavation,  is: 

"Payment  will  be  made  for  the  number  of  cubic  yards 
of  rock  measured  and  allowed  as  above  specified  at  the 
price  of  four  dollars  and  fifty  cents  ($4.50)  per  cu.  yd.,  meas- 
ured in  place.  Payment  for  rock  excavation  will  be  made 
in  addition  to  the  prices  bid  for  excavation." 

115.  Special  Specifications. — These  have  to  do  with  problems, 
methods  of  construction,  or  materials  peculiar  to  certain  contracts 
or  certain  portions  of  the  work.     It  frequently  occurs  that  the 
construction  of  sewerage  works  will  be  let  out  under  a  number  of 
contracts,  or  bids  will  be  called  for  on  different  alternatives    to 
which  the  entire  Advertisement,  Information  and    Instructions 
for  Bidders,  Proposal,  and  General  Specifications  are  applicable. 
The  special  specifications  will  apply  only  to  the  contract  in  ques- 
tion, e.g.,  in  some  work  done  under  the  direction  of  the  author, 
the  sewer  on  one  contract  came  within  twelve  .inches  of  the  surface 
of  a  highway.     The  special  specification  relating  to  this  piece  of 
construction,  was: 

"  Where  crossing  under  the  Chicago  Road  the  pipe  sewer 
shall  be  embedded  in  concrete  as  shown  on  the  contract 
drawings.  The  concrete  for  this  purpose  shall  be  mixed 
.in  the  proportions  of  one  (1)  part  cement,  three  (3)  parts 
fine  aggregate,  and  six  (6)  parts  coarse  aggregate.  Pay- 
ment for  the  concrete  so  used  will  be  made  at  the  unit 
price  stated  in  the  accompanying  Proposal." 

In  order  to  avoid  confusion  the  special  specifications  are  either 
incorporated  directly  in  the  Contract  form,  or  follow  the  Technical 
Specifications  and  are  grouped  according  to  the  contracts  to  which 
they  apply. 

116.  The  Contract. — The  contract  is  a  brief  instrument  which 
includes  a  simple  statement  of  the  obligations  of  each  party 
involved.     The  following  is  an  example  of  a  form  in  successful  use : 

CONTRACT 

This  agreement  made  and  entered  into  this   

day    of in    the    year    one    thousand    nine 

hundred  and by  and  between  the  City  of 

by  its  duly  constituted  or  elected  authorities  herein  acting 

for  the  City  of    without  personal  liability 

to  themselves,  party  of  the  first  part,  hereinafter  desig- 


THE  CONTRACT  231 

nated  as  the  City,  and 

party  of  the  second  part  hereinafter  designated  as  the 
Contractor. 

WITNESSETH,  that  the  parties  to  these  presents  each 
in  consideration  of  the  undertakings,  promises  and  agree- 
ments on  the  part  of  the  other  herein  contained,  have 
undertaken,  promised  and  agreed,  and  do  hereby  under- 
take, promise  and  agree,  the  party  of  the  first  part  for 

itself,  its  successors  and  assigns,  and  the  part of 

the  second  part  for and heirs,  executors, 

administrators  and  assigns  as  follows,  to-wit  : 

Art.  I.  To  be  bounden  by  all  the  articles  of  the  General, 
Technical,  and  Special  Specifications  applicable,  and  by 
the  terms  of  the  Advertisement,  Information  and  Instruc- 
tions for  Bidders,  Proposal  and  Contract  Drawings  hereto 
attached,  and  which  are  understood  and  acknowledged 
to  be  an  integral  part  of  this  contract. 

Art.  II.  The  work  to  be  completed  under  this  con- 
tract is 

Art.  III.  The  City  shall  pay  and  the  Contractor  shall 
receive  as  full  compensation  for  everything  furnished  and 
done  by  the  Contractor  under  this  contract,  including  all 
work  required  but  not  specifically  mentioned  in  the  follow- 
ing items,  and  also  for  all  loss  or  damage  arising  from  the 
nature  of  the  work  aforesaid,  or  from  the  action  of  the 
elements,  or  from  any  unforeseen  obstruction  or  difficulty 
encountered  in  the  prosecution  of  the  work  and  for  well 
and  faithfully  completing  the  work  as  herein  provided, 
as  follows: 

Then  follows  a  copy  of  the  Proposal  with  the  prices  bid.    The 
contract  closes  with  the  final  clause : 

In  witness  whereof  the  said  City  of ,  party 

of  the  first  part  have  hereunto  set  their  hands  and  seals, 
and  the  Contractor  has  also  hereunto  set  his  hand  and 
seal  and  the  party  of  the  first  part  and  the  Contractor 
have  executed  this  agreement  in  duplicate,  one  part  to 
remain  with  the  party  of  the  first  part  and  one  to  be  deliv- 
ered to  the  Contractor  this  day  of 

in  the  year  one  thousand  nine  hundred  and 

City  of 

Contractor  . 


232  CONTRACTS  AND  SPECIFICATIONS 

117.  The  Bond. — The  bond  called  for  in  the  Information  and 
Instructions 'for  Bidders  is  bound  in  the  pamphlet  following  the 
Contract.  No  uniform  practice  is  followed  in  the  amount  of  the 
bond  required.  It  varies  from  50  to  100  per  cent  of  the  contract 
price  and  may  be  stated  as  a  lump  sum  before  the  contract  price 
is  known.  There  is  a  possibility  that  the  Contractor  may  fail 
before  he  has  commenced  work  and  the  City  may  be  unable  to 
procure  another  contractor  to  take  up  the  work.  The  City  should 
then  be  protected  by  a  100  per  cent  bond.  Such  a  contingency  is 
remote.  The  Contractor  seldom  fails  until  work  is  well  under 
way,  and  other  contractors  are  usually  available,  although  the 
failure  of  one  contractor  tends  to  increase  the  bids  of  other  con- 
tractors for  the  same  work.  In  fixing  the  amount  of  the  bond 
the  judgment  of  the  Engineer  is  called  into  play  in  order  that  the 
amount  may  be  as  low  as  possible  in  fairness  to  the  Contractor, 
and  high  enough  to  protect  the  interests  to  the  City.  By  reducing 
the  amount  of  the  bond  the  expense  to  the  City  is  also  reduced 
as  the  City  ultimately  must  pay  its  cost. 

Upon  the  acceptance  of  the  bond  and  the  execution  of  the 
Contract,  the  Engineer's  duties  take  him  out  of  the  designing 
office  and  into  the  construction  field. 


CHAPTER  XI 
CONSTRUCTION 

118.  Elements. — The  principal  elements  in  construction  are: 
labor,    materials,    tools,    and    transportation.     The    lack    of    or 
inadequateness  of  any  one  of  these  detracts  from  the  effectiveness 
of  the  others.     The  engineer  should  assure  himself  of  the  com- 
pleteness of  his  plans  or  those  of  the  contractor  on  each  of  these 
points.     The  disposition  of  labor  and  the  handling  of  materials 
to  obtain  the  largest  amount  of  good  with  the  least  expenditure 
of  money  and  effort  are  problems  which  must  be  solved  by  the 
engineer  or  the  contractor  during  construction, 

WORK  OF  THE  ENGINEER 

119.  Duties. — The  duties  of  the  engineer  during  construction 
consist  in  giving  lines   and  grades;    inspecting  materials;  inter- 
preting the  contract,  specifications  and  drawings;   making  deci- 
sions when  unexpected  conditions  are  encountered;   making  esti- 
mates of  work  done;  collecting  cost  data;  making  progress  reports; 
keeping  records;  and  in  guarding  the  interests  of  the  City. 

120.  Inspection. — In    the    inspection    of    workmanship    and 
materials,  the  engineer  is  assisted  by  a  corps  of  inspectors  and 
assistants  who  act  under  his  direction.     The  duties  of  the  inspector 
are  to  be  present  at  all  times  that  work  is  in  progress  and  to  act 
for  the  engineer  in  enforcing   the   terms  of  the  contract,  the 
details  of  the  drawings,  and  the  tests  applicable  to  the  workman- 
ship and  materials  that  he  is  delegated  to  inspect.     He  should 
have  a  copy  of  the  contract,  or  that  portion  of  it  which  pertains 
to  his   work,   available  at  all  times.     He  should   examine  all 
materials  as  they  are  delivered  on  the  job  and  see  that  rejected 
materials  are  removed  at  once.     An  ordinary  recourse  of  some 
foremen  will  be  to  place  rejected  material  to  one  side  until  a  brief 
absence  of  the  inspector  will  present  the  opportunity  for  the  use 

233 


234  CONSTRUCTION 

of  the  rejected  material.  The  methods  to  be  followed  in  the 
inspection  of  materials  and  workmanship  should  be  such  as  to 
discover  discrepancies  between  the  specifications  and  the  materials 
delivered  or  the  work  done.  Other  duties  of  the  inspector  are: 
to  record  the  location  of  house  connections  or  to  drive  a  stake 
over  them  for  subsequent  location  by  the  engineer;  to  see  that 
plugs  are  put  in  the  branches  left  for  future  house  connections; 
to  inspect  the  workmanship  in  the  making  of  joints  in  pipe  sewers; 
to  protect  the  line  and  grade  stakes  from  displacement;  to  check 
the  size,  depth,  and  grade  of  sewers  and  elevations  of  special 
structures,  etc. 

Dishonest  and  unscrupulous  workmen  have  many  tricks  to 
get  by  the  inspector.  These  tricks  are  best  learned  by  experi- 
ence as  no  academic  list  can  impress  them  properly  on  the  memory. 
The  position  of  the  inspector  is  not  always  enviable.  He  must 
hold  the  respect  of  the  workmen,  of  the  contractor,  and  of  the 
engineer.  To  do  this  he  must  not  be  unreasonable  or  arbitrary 
in  his  decisions,  but  when  a  decision  is  once  made  he  must  be 
firm  in  following  up  its  enforcement.  He  must  be  careful  not  to 
give  directions  whose  fulfillment  he  cannot  enforce,  nor  for  which 
he  cannot  give  adequate  reason  to  his  superiors.  His  integrity 
must  never  be  questioned.  He  must  not  allow  himself  to  become 
under  obligations  to  the  contractor  by  the  acceptance  of  favors 
he  cannot  return  except  at  the  expense  of  his  employer,  yet  at 
the  same  time  he  must  not  appear  priggish  by  the  refusal  of  all 
favors  or  social  invitations.  In  brief  he  must  be  friendly  without 
being  intimate,  independent  without  being  aloof,  and  firm  without 
being  arbitrary. 

The  engineer  must  support  his  inspectors  in  their  decisions  or 
discharge  them  if  he  cannot. 

121.  Interpretation  of  Contract. — In  interpreting  the  contract, 
specifications  and  drawings,  the  engineer  is  supposedly  an 
impartial  arbiter  between  the  interests  of  the  city  and  the  con- 
tractor. His  decisions,  as  to  the  meaning  of  the  contract,  must 
be  founded  on  his  engineering  judgment,  and  should  aim  to  pro- 
duce the  best  results  without  demanding  more  from  the  contractor 
than,  in  his  honest  opinion,  it  is  the  intention  of  the  contract  to 
demand.  However  conscientiously  he  may  attempt  to  remain 
impartial,  and  in  spite  of  the  honesty  of  the  contractor,  his  posi- 
tion as  an  employee  of  the  city  will  almost  invariably  cause  him 


UNEXPECTED  SITUATIONS  235 

to  favor  the  city  in  his  decisions  on  close  points.  The  experi- 
enced contractor  knows  this  and  fixes  his  bid  accordingly,  the 
personality  of  the  engineer  sometimes  acting  as  an  important 
factor  in  the  amount  of  the  bid.  The  situation  arises  through 
the  character  of  the  contract,  and  not  through  a  lack  of  moral 
integrity  on  the  part  of  anyone  concerned. 

122.  Unexpected  Situations. — When  unexpected  or  uncertain 
conditions  are  encountered  in  construction  the  engineer  should 
visit  the  spot  at  once  and  should  advise  or  direct,  according  to  the 
terms  of  the  contract,  the  procedure  to  be  followed.     Such  condi- 
tions may  be  the  encountering  of  other  pipes,  quicksand,  rock,  etc. 
Each  case  is  a  problem  in  itself.     Water,  gas,  telephone  and  elec- 
tric wire  conduits  can  be  moved  above  or  below  the  sewer  being 
constructed   with   comparative  ease.     Other  sewers,   if  smaller, 
may  be  permitted  to  flow  temporarily  across  the  line  of  the  sewer 
under  construction  and  finally  discharge  into  the  completed  sewer, 
or  one  sewer  must  be  made  to  pass  under  the  other,  either  as  an 
inverted  siphon  or  by  changing  the  grade  of  one  of  the  sewers. 
Rock,  or  other  material  for  which  a  special  rate  of  payment  is 
allowed,  must  be  measured  as  soon  as  uncovered  in  order  to  avoid 
delaying  the  work  or  losing  the  record  of  the  amount  removed. 
When  quicksand  is  met  special  precautions  must  be  taken  to 
safeguard  the  sewer  foundation  and  to  insure  that  the  sewer  will 
remain  in  place  until  after  the  backfilling  is  completed.     These 
precautions  are  described  in  Art.  135. 

123.  Cost  Data  and  Estimates. — Cost  account  keeping  and 
the  making  of  monthly  or  other  estimates  are  closely  connected. 
Cost  accounts  are  of  value  in  estimating  the  amount  of  work  done 
to  date,  and  in  making  preliminary  estimates  of  the  cost  of  similar 
work.     Although  the  engineer  is  not  always  required  to  keep  such 
accounts,  they  are  usually  of  sufficient  value  to  pay  for  the  labor 
of  keeping  them.     Under  some  contracts  the  contractor's  accounts 
are  open  to  examination  by  the  engineer.     Usually,  however,  he 
must  depend  on  reports  from  the  inspectors  for  information  con- 
cerning the  man-hours  required  on  different  pieces  of  work,  and 
on  his  own  measurements  of  materials  used  and  his  knowledge  of 
their  unit  costs,  in  order  to  make  up  an  estimate  of  total  cost. 

The  measurement  of  a  completed  structure  and  a  summary  of 
the  materials  used  in  its  construction  may  act  as  a  check  on  the 
use  of  proper  materials  as  called  for  in  the  contract.  For  example, 


236 


CONSTRUCTION 


if  it  is  known  that  2,000  bricks  are  required  for  the  construction 
of  a  manhole  and  if  only  15,000  have  been  used  in  the  construction 
of  ten  manholes,  it  is  probable  that  some  or  all  of  the  manholes 
have  been  skimped.  Similar  conditions  may  show  in  the  pro- 
portions of  concrete,  backfilling  in  tunnels,  sheeting  to  be  left  in 
place,  etc. 

The  statement  of  a  few  principles  of  cost  accounting,  and  the 
illustration  of  a  few  blanks  in  use  should  be  sufficiently  suggestive 
to  lead  a  resourceful  engineer  in  the  right  direction.1  Costs  should 
be  divided  into  four  general  classifications:  labor,  materials, 
equipment,  and  overhead.  Labor  should  be  subdivided  under 
its  several  different  classifications  arranged  in  accordance  with 
rates  of  pay.  The  number  of  laborers  under  each  classification 
and  the  amount  of  work  done  per  day  should  be  recorded.  Fig.  86 
is  an  example  of  a  form  which  may  be  used  for  such  a  purpose. 


FOREMAN* 
Location..../  4th  Avenue.  ...No. 
[temized  Pay  Roll. 

s  DAILY  PAY  ROLL  REPORT. 
2    96"  Sewer                Date      August  7  1907 

Work  Done,                              Pay  Roll  Distributed. 

foreman  /  days  at 
Engineer  .  / 
Labor        1 
27.4 

;;        n.8 

Carts       "Y" 
Teams    2-10 
Hoister      1 
Water  boy  1 

Total  day's  pay 
roll  ......... 

4 
3 
3 
1 
1 

00 
50 
00 
75 
60 

I 

3 

% 

00 
50 
00 
95 
70 

General  Night  Watchman  and  Water  Boy  
Excavation  completed  to  station  18.40  Cost 
Sheeting                                              18.30     
Foundation  pl'nk"                           18.00     
Backfilling            "                           16.90    
Sheeting  Pulled   "                           17.10     
Concrete  Invert  4* 

2 
19 
8 

2 
3 
2 

25 
25 
25 
63 
00 
62 

! 

% 

00 
76 

3 
1 

2 

83 

00 
20 
00 
76 

10 

Brick  Invert         "                            
Concrete  Sides  \  "                          17.48     
Concrete  Roof  /  "                           

"29 

62 

Steel  Bars  set       "  ""*                     17.48   
Forms  set  to  station  17.48   
Forms,  Pipe  laid  to  station  

3 
7 

50 
25 

Manholes  built  to-day 

Other  items  Pump  

1 

3 
*83 

63 
20 

00 
10 

Teams  working  Cement  17.48   
Carts  working  Gravel  17.48   
Total  day's  pay  roll  

Signed  L.-—  W.-         Foreman. 

FIG.  86. — Foreman's  Daily  Payroll  Report. 

From  Engineering  and  Contracting,  1907. 

Materials  may  be  recorded  as  they  are  delivered  on  the  job,  as 
they  are  used,  or  in  both  cases.  Measurements  are  usually  easier 
to  make  at  the  time  of  delivery,  but  records  made  at  the  time 

1  Cost  Keeping  and  Management,  by  Gillette  and  Dana.  Practical  Cost 
Keeping  for  Contractors,  by  F.  R.  Walker.  Cost  Keeping  in  Sewer  Work, 
by  K.  O.  Guthrie  in  Eng.  Contracting,  Vol.  28,  p.  238,  1905.  Sewer  Con- 
struction Records  at  Scarsdale,  Eng.  News-Record,  Vol.  83,  p.  Ill,  1919. 


COST  DATA  AND  ESTIMATES 


237 


materials  are  used  are  more  serviceable.  For  example,  100 
barrels  of  cement  may  be  delivered  on  a  job  in  November,  50  of 
them  are  used  before  the  job  freezes  up  and  the  other  50  are  held 
over  until  spring.  It  would  be  misleading  to  charge  100  barrels 
used  in  November.  Fig.  87  is  a  form  in  use  for  an  inspector's 


FOREMAN'S  DAILY  MATERIAL  REPORT. 
Location....;  4th  Avenue  No.  2  96"  Date  August  7,  1907  

?ull  cement  bags  on  hand  lasl 
received  to-c 

Total 

night 

84 

lay       ""                           

160 

•244- 

Full  cement  used  to-day  on  c< 
"   b 

',',        1!           "         !'.       '/,   c< 

"        "                       "        "    m 
..           i*         ..        ..    p 

>nc  invert 

riclc     *' 

>nc.  sides  \    . 

166 

"     roof    )             

minting  up,  etc.*.  

1 

167 

Balance  on  hand  to-night 

87 

Smpty  cement  bags  on  hand  1 
Full  bags  used  to-day 

ast  night        

7 

167 

Pmntv  Viacr^  ^r*nt   in  fn»«1av 

Total  

164  .. 

' 

140 

Empty  bags  balance  on  hand  to-night- 

24-... 

Materials  received. 

From. 

J 
47- 

imounts. 
-4x6—  16 

_/*£_// 

-ft.... 

,.  ..12- 

-ft.;  

E"  "  N".  . 

1120' 

t*  roofers 

-1—16  ft. 
-}—  19-tt. 
-t—30-tt. 

Steel  Bars. 

Car  No.PRR  7284  

190- 
120- 

76- 

Sheeting  and  bracing  left  in 

None 



Forcnicin* 

FIG.  87. — Foreman's  Daily  Material  Report. 

From  Engineering  and  Contracting,  1907. 

report  on  materials.  The  total  cost  must  be  made  up  in  the 
office  from  these  records  and  a  knowledge  of  unit  costs. 

Equipment  consists  of  tools,  animals,  machinery,  and  appara- 
tus used  in  construction.  Only  equipment  that  is  actually  used 
should  be  charged  to  the  job  and  a  credit  should  be  made  at  the 
completion  of  the  job  for  the  fair  value  of  the  equipment  remain- 
ing after  the  completion  of  the  work. 

Overhead  charges  include  the  expense  of  the  office  force, 
superintendence,  and  miscellaneous  items  such  as  insurance,  rent, 


238  CONSTRUCTION 

transportation,  etc.,  which  cannot  be  charged  to  any  particular 
portion  of  the  work  but  are  equally  applicable  to  all  portions.  It 
happens  frequently  that  many  jobs  are  handled  in  the  same  main 
office.  The  division  of  overhead  becomes  more  difficult  and  is 
frequently  arranged  on  an  arbitrary  basis,  e.g.,  each  job  may  be 
charged  the  proportion  of  overhead  that  its  contract  price  bears 
to  the  total  contract  prices  being  performed  under  that  office. 
This  rule  may  be  modified  when  it  becomes  evident  that  some  job 
is  taking  distinctly  more  than  its  share  of  the  overhead. 

Estimates  of  work  done  in  any  period  can  be  made  with  the 
above  data  in  hand  by  subtracting  the  total  costs  of  the  work  up 
to  the  beginning  of  the  period  from  the  total  costs  up  to  the  end 
of  the  period.  Fig.  88  shows  a  sample  blank  from  the  final  esti- 
mate sheets  used  at  Scarsdale,  N.  Y. 

124.  Progress  Reports.1 — These  are  kept  by  the  engineer  in 
order  that  he  may  see  that  the  work  is  progressing  as  called  for  in 
the  contract,  and  any  portion  which  is  lagging  behind  without 
reason  may  be  pushed.     Such  reports  are  most  useful  when  the 
information  is  expressed  graphically,  as  the  eye  quickly  catches 
points  where  the  work  is  falling  behind  schedule. 

125.  Records. — The  contract  drawings  are  supposed  to  show 
exactly   where  and  how  construction  is  to  be  done.     Due  to 
unexpected  contingencies  changes  occur,  of  which  a  record  should 
be  made  and  preserved.     These  records  may  be  kept  in  a  form 
similar  to  the  contract  drawings,  or  if  the  changes  are  not  exten- 
sive, they  can  be  recorded  on  the  original  contract  drawings. 
The  location  of  house  and  other  connections  should  be  recorded 
in  a  separate  note  book  available  for  immediate  consultation. 
The  engineer  should  keep  a  diary  of  the  work  in  which  are  recorded 
events  of  ordinary  routine  as  well  as  those  of  special  interest  and 
importance.     This  diary  should  be  illustrated  by  photographs 
showing  the  condition  of  the  streets  before  and  after  construction, 
methods  of  construction,  accidents,  etc.     Such  accounts  are  of 
great  value  in  defending  subsequent  litigation  and  their  existence 
sometimes  prevents  litigation.     A  contractor  may  wait  a  year  or 
so  after  the  completion  of  a  piece  of  work  until  the  engineer  and 
other  city  officials  have  broken  their  connection  with  the  city. 
Suit  is  then  brought  against  the  city  and  unless  good  records  are 

1  See  Planning  and  Progress  on  a  Big  Construction  Job,  by  Chas  Penrose, 
Eng.  News-Record,  Vol.  84,  1920,  pp.  554  and  627. 


COST  DATA  AND  ESTIMATES 


239 


24-"  VIT.     SEWER 
.6  FT.  TO    8  FT.  DEEP,  INCLUDING  &  FT. 

1914                                     LOCATION                               SCHEDULE    FEET 

PRICE 

AMOUNT 

tfj^\7\&£vctnMH*+&M.H*S    (&*t&4.'&<±) 
"This  siqn  indioates  that  the  item  has  been  inch 

A 
tded  in  t 

2GO.O  f  203  ft  406.  U* 
he  monthly  estimate 

V-BRANCHES   ON    24"  PIPE 

LOCATION                                SCHEDULE  NUMBER 

PRICE  AMOUNT 

fra.7*2p552«*5  <***4 

A 

6 

4*c 

:  24. 

00* 

gr 
1914  |S 

MANHOLES    6FTTO8FT 
LOCATION                               SCHEDULE 

PRICE  AMOUNT 

M, 

5 

^^^^^^ 

A 

SD.OO  .          J~0. 

CO* 

1914  §o 

MANHOLE    DEPTHS     EXCEEDING    6  FT. 

LOCATION                                SCHEDULE    FEET    PRICE  AMOUNT 

?/  [&eto£^^£*^4|*$^£«* 

K           4-. 

3     p.fffff     /2. 

10* 

•191.4 

ROCK 
IN     SEWER     TRENCH 
LOCATION                                     SCHEDULE   CO.YDS  PRICE  AMOUNT 

4* 

2^PfF^2~25 

^'        "J 

4-1  ZfS  L    //4. 

// 

| 

1914     £ 

fe 

QC     . 

R  0  'C    K 

IN     MANHOLE     EXCAVATION 

LOCATION                            SCHEDULE   CU.  YDS.  PRICE  AMOUNT 

*4* 

^^^^^^^B, 

F         /4 

*         t    •'. 
Of  Z.15^  o      JO. 

** 

o  £  < 
Rj*l 
.EN6TH55^2 


CONTINGENT      EXTRAS 

STAN  DARD-A-SECTION 
CONCRETE     FOR    SEWERS 
LOCATION  .    SCHEDULE  CU.  YDS.  PRICE  AMOUNT 


Z73.8  24  3 


"A 


1914 


CONTINGENT     EXTRAS 

EDULE  BOARD  Ff.  PRICE  AMOUNT 


//*  o. 


31.  ?Q 


FIG.  88. — Samples  of  Cost  Record  Forms. 

From  Engineering  and  Contracting,  1909. 


240  CONSTRUCTION 

available  the  administration  may  be  forced  to  buy  the  claimant 
off  or  may  elect  to  enter  court,  only  to  be  beaten. 

EXCAVATION 

126.  Specifications. — The  following  abstracts  have  been  taken 
from  the  specifications  on  Excavation  by  the  Baltimore  Sewerage 
Commission  as  illustrative  of  good  practice.  In  conducting  the 
work  the  contractor  shall: 

.  .  .  remove  all  paving,  or  grub  and  clear  the  surface 
over  the  trench,  whenever  it  may  be  necessary  and  shall 
remove  all  surface  materials  of  whatever  nature  or  kind. 
He  shall  properly  classify  the  materials  removed,  separat- 
ing them  as  required  by  the  Engineer;  and  shall  properly 
store,  guard,  and  preserve  such  as  may  be  required  for 
future  use  in  backfilling,  surfacing,  repaving  or  otherwise. 
All  macadam  material  removed  shall  be  separated  and 
graded  into  such  sizes  as  the  Engineer  may  direct  and 
materials  of  different  sizes  shall  be  kept  separate  from  each 
other  and  from  any  and  all  other  materials. 

All  the  curb,  gutter,  and  flag-stones  and  all  paving 
material  which  may  be  removed,  together  with  all  rock, 
earth  and  sand  taken  from  the  trenches  shall  be  stored  in 
such  parts  of  the  carriageway  or  such  other  suitable  place, 
and  in  such  manner  as  the  Engineer  may  approve.  The 
Contractor  shall  be  responsible  for  the  loss  of  or  damage 
to  curb,  gutter  and  flag-stones  and  to  paving  material 
because  of  careless  removal  or  wasteful  storage,  disposal, 
or  use  of  the  same. 

.  .  .  When  so  directed  by  the  Engineer  the  bottom  of 
the  trench  shall  be  excavated  to  the  exact  form  of  the 
lower  half  of  the  sewer  or  of  the  foundation  under  the 
sewer. 

The  bottom  width  of  the  trench  for  a  brick  or  concrete 
sewer  shall  be  ...  not  less  in  any  case  than  the  overall 
width  of  the  sewer,  as  shown  on  the  plans.  In  case  the 
trench  is  sheeted  this  minimum  width  will  be  measured 
between  the  interior  faces  of  the  sheeting  as  driven,  but  in 
no  case  shall  bracing,  stringers,  or  waling  strips  be  left 
within  any  portion  of  the  masonry  of  the  sewer  except  by 
permission  of  the  Engineer;  and  such  braces,  stringers 
and  waling  strips  shall  not,  in  any  case,  be  allowed  to 
remain  within  the  neat  lines  of  the  masonry  as  shown  on 
the  plans.  In  case  that  the  distance  between  faces  of  the 
sheeting  is  less  than  that  called  for  by  the  width  of  the 


SPECIFICATIONS  241 

sewer  to  be  laid  in  the  trench,  the  Engineer  may  direct  the 
sheeting  to  be  drawn  and  redriven,  or  otherwise  changed 
and  altered;  or  he  may  direct  that  the  sewer  be  reinforced 
in  such  manner  and  to  such  an  extent  as  he  may  deem 
necessary  without  compensation  to  the  Contractor,  even 
though  such  narrower  trench  was  not  caused  by  negli- 
gence or  other  fault  on  the  part  of  the  Contractor. 

Trenches  for  vitrified  pipe  shall  be  at  all  points  at  least 
six  inches  wider  in  the  clear  on  each  side  than  the  greatest 
external  width  of  the  sewer,  measured  over  the  hubs  of  the 
pipe  .  .  .  Bell  holes  shall  be  excavated  in  the  bottoms  of 
trenches  for  vitrified  pipe  sewers  wherever  necessary. 

Not  more  than  three  hundred  feet  of  trench  shall  be 
opened  at  any  one  time  or  place  in  advance  of  the  com- 
pleted building  of  the  sewer,  unless  by  written  permission 
of  the  Engineer  and  for  a  distance  therein  specified.  .  .  . 

The  excavation  of  the  trench  shall  be  fully  completed 
at  least  twenty  feet  in  advance  of  the  construction  of  the 
invert,  unless  otherwise  ordered. 

During  the  progress  of  construction  the  Contractor  will 
be  required  to  preserve  from  obstruction  all  fire  hydrants 
and  the  carriageway  on  each  side  of  the  line  of  the  work. 

The  streets,  cross  walks,  and  sidewalks  shall  be  kept 
clean,  clear,  and  free  for  the  passage  of  carts,  wagons,  car- 
riages and  street  or  steam  railway  cars,  or  pedestrians, 
unless  otherwise  authorized  by  special  permission  in  writ- 
ing from  the  Engineer.  In  all  cases  a  straight  and  con- 
tinuous passageway  on  the  sidewalks  and  over  the  cross 
walks  of  not  less  than  three  feet  in  width  shall  be  pre- 
served free  from  all  obstruction. 

Where  any  cross  walk  is  cut  by  the  trench  it  shall  be 
temporarily  replaced  by  a  timber  bridge  at  least  three  feet 
wide,  with  side  railings,  at  the  Contractor's  expense.  The 
placing  of  planks  across  the  trench  without  proper  means 
of  connection  or  fastenings,  or  pipe  or  other  material,  or 
the  using  of  any  other  makeshift  in  place  of  properly  con- 
structed bridges,  will  not  be  permitted. 

This  is  equally  applicable  to  certain  wagon  bridges  to  be  fixed 
upon  by  the  Engineer,  on  the  basis  of  traffic  requirements. 

In  streets  that  are  important  thoroughfares  or  in  narrow 
streets  the  material  excavated  from  the  first  one  hundred 
feet  of  any  opening  or  from  such  additional  length  as  may 
be  required,  shall  upon  the  order  of  the  Engineer,  be 
removed  by  the  Contractor,  as  soon  as  excavated.  The 
material  subsequently  excavated  shall  be  used  to  refill  the 
trench  where  the  sewer  has  been  built. 


242  CONSTRUCTION 

The  preceding  specifications  are  applicable  to  open-trench 
excavation.  Rigid  restrictions  are  placed  about  tunneling 
because  of  the  greater  difficulty  of  doing  good  work,  the  greater 
danger  to  life  and  property  and  the  possibility  of  later  surface 
subsidence  if  the  backfilling  is  done  improperly.  A  common 
clause  in  specifications  is: 

All  excavations  for  sewers  and  their  appurtenances 
shall  be  made  in  open  trenches  unless  written  permission 
to  excavate  in  tunnel  shall  be  given  by  the  Engineer. 

127.  Hand  Excavation. — Earth  excavation  by  pick  and  shovel 
is  the  simplest  and  most  primitive  mode  of  excavation.  Only 
small  jobs  are  handled  in  this  manner  in  order  to  save  the  invest- 
ment necessary  in  machines  or  the  expense  of  hiring  and  moving 
one  to  the  work.  The  tools  used  in  the  hand  excavation  of  trenches 
are:'  picks,  pickaxes,  long-handled  and  short-handled  pointed 
shovels,  square-edged  long-  and  short-handled  shovels,  scoop 
shovels,  axes,  crowbars,  rock  drills,  mauls,  sledges,  etc.  The 
excavating  gangs  are  divided  up  into  units  of  20  to  50  men  under 
one  foreman  or  straw  boss,  and  among  the  men  may  be  a  few 
higher  priced  laborers  who  set  the  pace  for  the  others.  Each 
laborer  on  excavation  should  be  provided  with  a  shovel,  the 
style  being  dependent  on  the  character  of  the  material  being 
excavated  and  the  depth  of  the  trench.  In  stiff  material  and 
deep  trenches  requiring  the  lifting  of  the  material  in  the  shovel, 
long-handled  pointed  shovels  should  be  used.  In  loose  sandy 
material  loaded  directly  into  buckets  short-handled,  square 
pointed  shovels  are  satisfactory.  Picks  are  used  in  cemented 
gravels  or  where  hard  obstructions  prevent  cutting  down  with 
the  edge  of  the  shovel.  Very  stiff  but  not  hard  material  can  be 
cut  out  in  chunks  with  a  pickaxe  and  thrown  from  the  trench  or 
into  a  bucket  with  a  scoop  shovel.  Scoop  shovels  are  also  useful 
in  wet  running  quicksand.  The  number  of  picks,  axes,  crowbars, 
and  other  tools  must  be  proportioned  according  to  the  material 
being  excavated.  Under  the  worst  conditions  of  excavation  in  a 
hard  cemented  gravel  it  may  be  necessary  to  provide  each  man 
with  a  pick  as  well  as  a  shovel,  whereas  in  sand  only  a  shovel  is 
necessary.  Two  or  three  crowbars,  axes,  a  length  of  chain,  two 
or  .three  screw  jacks,  etc.,  are  provided  per  gang  in  case  of  an 
unexpected  encounter  with  an  obstruction  in  the  trench,  such  as 
a  boulder,  a  tree  stump,  a  length  of  pipe,  etc. 


HAND  EXCAVATION 


243 


In  laying  out  the  work  the  foreman  marks  the  outlines  of  the 
trench  on  the  ground  by  means  of  a  scratch  made  with  a  pick, 
chalk  marks,  tape,  or  other  devices.  These  marks  are  measured 
from  offset  or  center  stakes  set  by  the  engineer.  Center  stakes 
are  less  conducive  to  error  but  are  more  likely  to  be  disturbed 
before  use  than  are  offset  stakes,  but  careless  foremen  make  more 
errors  with  offset  than  with  center  stakes.  The  inspector  should 
assist  or  be  present  at  the  laying  out  of  the  trench.  After  the 
trench  has  been  laid  out  each  laborer  should  be  given  a  certain 
specific  portion  of  it  to  dig  and  this  portion  is  marked  out  on  the 
ground.  In  this  way  a  check  can  be  kept  upon  the  performance 
of  each  laborer  and  the  knowledge  of  this  fact  tends  to  a  uni- 
formly better  performance.  The  amount  of  work  that  can  be 
performed  by  one  man  with  a  pick  and  shovel  is  as  shown  in 
Table  49.  Some  men  may  exceed  these  rates,  many  will  not 
attain  them.  The  allotted  task  must  be  gaged  on  the  character 
of  the  ground  in  order  that  the  tasks  may  be  equal  and  a  spirit  of 
competition  fostered.  The  hard  worker  will  set  the  pace  for  the 
lazy  man.  Some  contractors  have  adopted  the  expedient  of  dis- 
missing laborers  for  the  day  as  soon  as  the  allotted  task  is  done. 

TABLE  49 

AMOUNT  OF  MATERIAL   MOVED    BY  ONE  MAN  WITH  A  PICK  AND  SHOVEL 
(From  H.  P.  Gillette) 


Material 

Cubic  Yard 
per  Hour 

Material 

Cubic  Yard 
per  Hour 

Hardpan  

0.33 

Sand  

1.25 

Common  earth  

O.Sto  1.2 

Sandy  soil  

0  8  to  1  2 

Stiff  clay  
Clay 

0.85 
1  00 

Clayey  earth  
Sandy  soil  (frozen) 

1.3 

0  75 

The  opening  of  the  trench  may  be  facilitated  by  breaking 
ground  with  a  plow.  In  hard  ground  or  on  paved  roads  it  may 
be  necessary  to  cut  through  the  surface  crust  with  a  hammer  and 
drill,  although  in  some  cases  a  plow  can  be  used  successfully. 
Frozen  ground  can  be  thawed  by  building  fires  along  the  line  of 
the  trench,  or  greater  economy  may  be  achieved  by  placing  steam 
pipes  along  the  surface  with  perforations  about  every  18  inches 


244  CONSTRUCTION 

and  either  boxing  them  on  the  top  and  sides  .or  burying  them  in 
the  frozen  earth  with  a  covering  of  sand.  Another  arrangement 
is  to  blow  steam  into  a  line  of  bottomless  boxes  in  which  each  box 
is  about  8  feet  long.  Holes  are  left  in  the  top  of  the  boxes  into 
which  the  pipe  is  shoved,  and  after  its  withdrawal  the  holes  are 
covered.  Blasting  of  frozen  earth  is  sometimes  successful  but 
cannot  be  resorted  to  in  built  up  districts  where  it  is  unsafe  unless 
properly  controlled.  Once  the  frost  crust  is  broken  through  it 
can  be  attacked  from  below  and  frequently  broken  down  by 
undermining. 

A  laborer  cannot  dig  and  raise  the  earth  much  more  than  to 
the  height  of  his  head,  and  preferably  not  quite  so  high,  without 
tiring  quickly.  After  the  trench  has  passed  a  depth  of  4  feet  he 
cannot  throw  the  earth  clear  of  the  trench.  An  additional  laborer 
is  needed  then  at  the  surface  to  throw  the  earth  back.  He  should 
shovel  the  earth  from  a  board  platform  placed  at  the  edge  of  the 
trench  as  a  protection  to  the  bank.  When  the  trench  passes  the 
6-foot  depth  a  staging  is  put  in  about  4  feet  from  the  top  on  which 
the  lowest  laborer  piles  his  materials.  It  is  then  passed  up  to  the 
surface  by  a  second  laborer  on  the  staging,  and  a  third  laborer  on 
the  surface  throws  the  material  back  clear  of  the  trench.  Stag- 
ings are  put  in  about  every  5  or  6  feet  for  the  full  depth  of  the 
trench. 

When  the  trench  has  come  within  half  the  diameter  of  the 
pipe  of  the  final  grade,  if  the  material  is  sufficiently  firm,  the 
remainder  of  the  trench  should  be  cut  to  conform  to  the  shape  of 
the  lower  half  of  the  outside  of  the  pipe,  with  proper  enlargements 
for  each  bell. 

128.  Machine  Excavation. — On  work  of  moderately  large 
magnitude  excavation  by  machine  is  cheaper  than  by  pick  and 
shovel  alone.  In  comparing  the  cost  of  excavation  by  the  two 
methods  all  items  such  as  sheeting,  pipe  laying,  backfilling,  etc., 
should  be  included,  since  these  items  will  be  affected  by  the  method 
of  excavation.  The  cost  of  setting  up  and  reshipping  the  machine 
must  be  included  as  this  is  frequently  the  item  on  which  the  use 
of  the  machine  depends.  Because  of  the  cost  of  setting  up  and 
shipping,  which  must  be  distributed  over  the  total  number  of 
yards  excavated,  the  cost  per  cubic  yard  of  excavating  by  machine 
varies  with  the  number  of  cubic  yards  excavated.  The  point  of 
economy  in  the  use  of  a  machine  is  reached  when  the  cost  by  hand 


MACHINE  EXCAVATION  245 

and  by  machine  are  equal.  For  all  work  of  greater  magnitude, 
excavation  by  machine  will  prove  cheaper.1  Items  favoring  the 
use  of  machinery  which  may  cause  its  adoption  for  small  jobs  are: 
its  greater  speed,  reliability,  ease  in  handling,  economy  in  sheet- 
ing, economy  in  labor,  and  small  amount  of  space  needed  making 
it  useful  in  crowded  streets.  Continuous  bucket  machines,  drag 
lines,  and  occasionally  steam  shovels  are  not  adapted  to  conditions 
where  rocks,  pipes  and  other  underground  obstacles  are  frequently 
met. 

The  following  problem  is  an  example  of  the  work  necessary  in 
making  a  comparison  of  the  relative  economy  of  machine  and 
hand  excavation: 

It  is  assumed  that  a  man  can  excavate  15  feet  of  trench 
30  inches  wide  and  8  feet  deep  in  10  hours.  He  receives 
55  cents  per  hour  for  his  work.  A  machine  costing  $10,000 
has  a  life  of  6  years.  It  can  be  kept  busy  150  days  in  the 
year.  When  operating  it  costs  $1.25  per  hour  for  the 
operator,  fuel  and  repairs.  It  will  excavate  800  linear  feet 
of  30  inch  trench  to  a  depth  of  8  feet  in  10  hours.  It  is 
assumed  that  capital  is  worth  10  per  cent  on  such  a  venture 
and  that  the  sinking  fund  will  draw  10  per  cent.  If  the 
cost  of  moving  and  setting  up  the  machine  is  $1,800,  how 
many  cubic  yards  of  excavation  must  there  be  to  make 
excavation  by  machine  economical.  Costs  of  sheeting, 
pumping,  etc.,  are  assumed  to  be  the  same  for  machine  or 
hand  work. 

Solution. — For  hand  work  the  man  excavated  1.11 
cubic  yard  per  hour  at  55  cents.  The  relative  cost  of  hand 
excavation  is  then  50  cents  per  cubic  yard. 

The  cost  of  machine  work  will  be  divided  into :  interest 
on  first  cost;  operation  and  repairs;  and  sinking  fund  for 
renewal.  The  interest  on  the  first  cost  of  $10,000  at 
10  per  cent  is  $1,000  per  year.  The  machine  works  1,500 
hours  in  the  year.  Therefore  the  cost  per  hour  is  $0.67. 

The  sinking  fund  payment,  as  found  from  sinking  fund 
tables  or  the  accumulation  of  $10,000  in  6  years,  is  $1,300 
per  year  or  per  hour  for  1,500  hours  is  $0.87. 

The  cost  of  operation  per  hour  is  given  as  $1.25. 

The  total  cost  per  hour  is  therefore  $2.79. 

The  machine  excavated  59.3  cubic  yards  per  hour 
which  makes  the  cost,  exclusive  of  moving,  equal  to  $0.47 

1  See  also  "  Ownership  and  Operation  of  Trench  Excavators  by  the 
Water  Department  of  Baltimore,"  by  V.  B.  Seims,  presented  before  Am. 
Water  Works  Association,  June  9,  1921. 


CONSTRUCTION 

per  cubic  yard.  In  order  to  equalize  the  cost  of  machine 
and  hand  excavation  the  cost  of  moving  the  machine  must 
be  divided  among  a  sufficient  number  of  cubic  yards  so  that 
the  cost  per  cubic  yard  shall  be  3  cents.  The  cost  of  moving 
is  given  as  $1,800.  This  amount  divided  among  60,000 
cubic  yards  equals  3  cents  per  cubic  yard.  Therefore  the 
job  must  provide  at  least  60,000  cubic  yards  of  excavation 
in  order  that  the  use  of  the  machine  shall  be  justifiable 
from  the  viewpoint  of  economy  alone. 

129.  Types  of  Machines. — Machines  particularly  adapted  to 
the  excavation  of  sewer  and  water  pipe  trenches  are  of  four  types: 
(1)  continuous  bucket  excavators;  (2)  oVerhead  cable  way  or  track 
excavators;  (3)  steam  shovels;  and  (4)  boom  and  bucket  excava- 
tors.    Other  types   of  excavating   machinery   can  be  used   for 
sewer  trenches  under  special  conditions.     Machines  are  ordinarily 
limited  to  a  minimum  width  of  trench  of  22  inches.     Between 
widths  of  22  inches  and  36  inches  the  limit  of  depth  for  the  first 
class  of  machines  is  about  25  feet.     For  other  types  of  machines 
there  is  no  definite  limit,  though  the  economical  depth  for  open 
cut  work  seldom  exceeds  40  feet. 

130.  Continuous    Bucket     Excavators. — Continuous     bucket 
excavators  are  of  the  types  shown  in  Figs.  89  and  90.     The 
buckets  which  do  the  digging  and  raising  of  the  earth  may  be 
supported  on  a  wheel  as  in  Fig.  89  or  on  an  endless  chain  as  in 
Fig.  90.     The  support  of  the  wheel  or  endless  chain  can  be  raised 
or  lowered  at  the  will  of  the  operator  so  as  to  keep  the  trench  as 
close  to  grade  as  can  be  done  by  hand  work.     In  some  machines 
the  shape  of  the  buckets  can  be  made  such  as  to  cut  the  bottom  of 
the  trench,  in  suitable  material,  to  the  shape  of  the  sewer  invert. 
In  operation,  the  buckets  are  at  the  rear  of  the  machine  and 
revolve  so  that  at  the  lowest  point  in  their  path  they  are  traveling 
forward.     The  excavated  material  is  dropped  on  to  a  continuous 
belt  which  throws  it  on  the  ground  clear  of  the  trench,  into  dump 
wagons,  or  on  to  another  continuous  belt  running  parallel  with  the 
trench  to  the  backfiller,  by  means  of  which  the  excavated  material 
is  thrown  directly  into  the  backfill  without  rehandling.     The 
body  of  the  machine  supporting  the  engine  travels  on  wheels 
ahead  of  the  excavation  and  is  kept  in  line  by  means  of  the  pivoted 
front    axle.     When    obstacles    are    encountered    the    excavating 
wheel  or  chain  is  raised  to  pass  over  the  obstacle,  and  allowed  to 
dig  itself  in  on  the  other  side. 


CONTINUOUS  BUCKET  EXCAVATORS 


247 


FIG.  89. — Buckeye  Wheel  Excavator. 

Courtesy,  Buckeye  Traction  Ditcher  Co. 


FIG.  90. — Buckeye  Endless-chain  Excavator. 
Courtesy,  Buckeye  Traction  Ditcher  Co. 


248 


CONSTRUCTION 


Wheel  excavators  are  not  adapted  to  the  excavation  of  sewer 
trenches  over  3  to  4  feet  in  width  and  6  to  8  feet  in  depth.  The 
endless-chain  excavators  are  suitable  for  depths  of  25  feet  with 
widths  from  22  to  72  inches,  and  due  to  the  arrangement  permit- 
ting buckets  to  be  moved  sideways  they  will  cut  trenches  of  differ- 
ent widths  with  the  same  size  buckets.  This  is  an  advantage 
where  there  are  to  be  irregularities  in  the  width  of  the  trench 
such  as  for  manholes  or  changes  in  size  of  pipe.  With  excavating 

machines  pipe  can  be  laid 
within  3  feet  of  the  moving 
buckets  and  the  trench  back- 
filled immediately,  thus  mak- 
ing an  appreciable  saving 
in  the  amount  of  sheet- 
ing. In  the  construction  of 
trenches  for  drain  tile  at 
Garden  Prairie,  Illinois,  the 
sheeting  was  built  in  the 
form  of  a  box  or  shield, 
fastened  to  the  rear  of  the 
machine  and  pulled  along 
after  it  as  is  shown  in 
Fig.  91. 

The  performance  of  this  type  of  excavating  machine  under 
suitable  conditions  is  large.  A  remarkable  record  was  made  by 
Ryan  and  Co.  in  Chicago,1  with  an  excavating  machine.  1338 
feet  of  32-inch  trench  were  excavated  to  an  average  depth  of  8§ 
feet  in  7  hours,  or  an  average  of  160  cubic  yards  per  hour.  More 
could  have  been  accomplished  if  it  had  not  been  for  delays  in 
supplies.  Another  crew  at  Greeley,  Colorado,2  with  a  Buckeye 
endless-chain  ditcher  weighing  17  tons  and  costing  $5200,  averaged 
232  cubic  yards  per  day  for  300  days,  and  the  cost  was  10.7  cents 
per  cubic  yard.  A  15-ton  Austin  excavator  can  be  expected  to 
remove  300  to  500  cubic  yards  per  day. 

The  cost  of  operation  of  the  machines  is  made  up  of  items 
listed  in  Table  50.     The  figures  given  are  merely  suggestive. 
In  making  a  comparison  of  the  cost  of  hand  and  machine 


FIG.  91. — Movable  Sheeting  Fastened  to 

Traction  Ditcher. 
From  Eng.  News-Record,  Vol.  82,  1919,  p.  740. 


1  Eng.  and  Contracting,  Vol.  48,  1917,  p.  492. 

2  Earth  Excavation  by  A.  B.  McDaniel. 


CONTINUOUS  BUCKET  EXCAVATORS 


249 


TABLE  50 
COST  OF  OPERATING  DITCHING  MACHINE 


Per  Day 

Total 

Labor  : 
1  Operator  at  $150  per  month                           

$6  00 

1  Assistant  Operator  at  $120  per  month 

4  00 

4  Laborers  at  $4  .  00  per  day  

16.00 

fcofi  no 

Fuel: 
20  Gallons  of  gasoline  at  28  cents  

5.60 

5.60 

Miscellaneous  : 
Oil  waste  etc                                                   

1.20 

Repairs  and  maintenance 

10  00 

Interest,  6  per  cent  on  $10,000  for  150  days  
Depreciation,  200  working  days  per  year  and  an 
8  year  life 

4.00 
11  11 

26  31 

Total  cost  per  day 

$57  91 

TABLE  51 

COMPARISON  OF  COST  OF  HAND  EXCAVATION  AND  MACHINE  EXCAVATION 
WITH  CONTINUOUS-BUCKET  EXCAVATOR 


Hand  Work 

Per  Day, 
Dollars 

Machine  Work 

Per  Day, 
Dollars 

Foreman 

4  00 

Engineer  

4  00 

Timberman 

3  00 

Fireman 

2  50 

Helper       

2.50 

Coal  

5  00 

40  Laborers  at  $2  00 

80  00 

Team  

4  00 

Foreman 

4  00 

Pipe  layer  
Helper      .  . 

3.00 
2  50 

2  Teams  backfilling  
2  Helpers  
Interest,  depreciation  and 
repairs  

8.00 
4.00 

10.00 

Total               

95  00 

Total 

54  50 

250  CONSTRUCTION 

excavation  the  figures  given  in  Table  51  are  from  "  Excavating 
Machinery  "  by  McDaniel,  who  quotes  the  cost  of  machine  exca- 
vation from  the  manufacturers  of  the  Parsons  machine  issued  as 
the  result  of  several  years'  experience  with  their  excavator.  In 
the  comparison  the  hand  crew  is  assumed  to  dig  315  linear  feet 
of  trench  28  inches  wide  by  12  feet  deep  in  a  day  of  10  hours. 
This  assumes  that  each  man  will  excavate  7  cubic  yards  per  day. 
The  machine  is  assumed  to  excavate  250  feet  of  the  same  trench. 
The  comparison  indicates  that  an  excavator  will  work  at  about 
50  per  cent  of  the  cost  of  hand  excavation,  if  the  cost  of  moving 
the  machine  is  not  included. 


FIG.  92. — Carson  Excavating  Machine  on  Trench  Excavation  in  South  Mil- 
waukee. 

Courtesy,  Mr.  C.  F.  Henning. 

131.  Cableway  and  Trestle  Excavators. — Cableway  and 
trestle  excavators  are  most  suitable  for  deep  trenches  and  crowded 
conditions.  They  should  not  be  used  for  trenches  much  less  than 
8  feet  in  depth.  They  differ  from  the  continuous-bucket  excava- 
tors in  that  the  actual  dislodgment  of  the  material  is  done  by  pick 
and  shovel,  the  excavated  material  being  thrown  by  hand  into  the 
buckets  of  the  machine.  A  machine  of  the  Carson  type  is  shown 
in  Fig.  92.  The  machine  consists  of  a  series  of  demountable 
frames  held  together  by  cross  braces  and  struts  to  form  a  semi- 
rigid structure.  An  I  beam  or  channel  extending  the  length  of 


CABLEWAY  AND  TRESTLE  EXCAVATORS  251 

the  machine  is  hung  closely  below  the  top  of  the  struts.  The 
lower  flange  of  this  beam  serves  as  a  track  for  the  carriages  which 
carry  the  buckets.  All  the  carriages  are  attached  to  each  other 
and  to  an  endless  cable  leading  to  a  drum  on  the  engine.  This 
cable  serves  to  move  the  buckets  along  the  trench.  The  buckets 
are  attached  to  another  cable  which  is  wound  around  another 
drum  on  the  engine  and  serves  to  lower  or  raise  all  the  buckets  at 
the  same  time.  In  operation  there  are  always  at  least  two  buckets 
for  each  carriage,  one  in  the  trench  being  filled  and  the  other  on 
the  machine  being  dumped.  There  should  be  a  surplus  of  buckets 
to  replace  those  needing  repairs. 

The  machines  may  be  from  200  to  350  feet  in  length,  and  the 
number  of  buckets  which  can  be  lifted  at  one  time  varies  from 
one  to  a  dozen  or  more.  On  trenches  over  5  to  6  feet  in  width  a 
double  line  of  buckets  is  sometimes  used.  The  entire  machine 
rests  on  rollers  and  straddles  the  trench.  It  is  moved  along  the 
trench  by  its  own  power,  either  by  gearing  or  chains  attached  to 
the  wheels,  or  by  a  cable  attached  to  a  dead-man  ahead. 

The  Potter  trench  machine  differs  from  the  Carson  in  that 
only  2  buckets  are  used  at  a  time  and  these  are  carried  on  a  car 
which  travels  on  a  track  on  top  of  the  trestle.  The  movement  of 
the  buckets  and  the  car  are  controlled  by  2  dump  men  who 
ride  on  the  car  and  who  can  raise  or  lower  the  buckets  inde- 
pendently. 

The  organization  needed  to  operate  these  machines  is:  a 
lockman  who  locks  and  unlocks  the  buckets  on  the  cable,  a 
dumper,  as  many  shovelers  as  there  are  buckets  on  the  machine, 
and  an  engineman  who  is  usually  his  own  fireman.  From  50  to 
400  cubic  yards  of  material  can  be  excavated  in  a  day  with  one  of 
these  machines,  dependent  on  the  character  of  the  material  and 
the  depth  of  the  trench.  H.  P.  Gillette  in  his  Handbook  of  Cost 
Data  reports  that  about  190  cubic  yards  were  excavated  per  day 
with  a  Potter  machine.  The  machine  was  370  feet  long.  Six 
f-yard  buckets  were  used,  4  in  the  trench  and  2  on  the  carrier. 
The  trench  was  10J  feet  wide  and  18  feet  deep  in  wet  sand  and 
soft  blue  clay.  The  organization  consisted  of  an  engineman,  a 
fireman,  2  dumpmen  on  the  carrier,  and  from  17  to  21  excavating 
laborers  depending  on  the  kind  and  the  amount  of  the  excavation. 
In  general  the  capacity  of  such  machines  is  limited  by  the  amount 
of  material  which  can  be  shoveled  into  them  by  hand. 


252  CONSTRUCTION 

132.  Tower  Cableways. — These  are  essentially  of  the  same 
class  as  the  trestle  cableway  machines.     They  differ  in  that  the 
carriage  supporting  the  buckets  travels  on  a  cable  suspended 
between  2  towers  instead  of  on  a  track  supported  on  a  trestle. 
As  a  rule  only  one  bucket  is  handled  in  the  machine  at  a  time. 
They  are  used  in  sewer  work  only  in  exceptional  cases  as  the 
towers  must  be  taken  down  and  re-erected  each  time  that  there 
is  an  advance  in  the  trench  greater  than  the  distance  between  the 
towers. 

133.  Steam  Shovels. — The  use  of  steam  shovels  for  the  exca- 
vation of  sewer  trenches  is  becoming  more  prevalent  because  of 
their  growing  dependability  and  durability  as  compared  with  other 
machines,  their  adaptability  for  small  trenches,  and  the  relatively 
large  number  of  widely  different  uses  to  which  they  can  be  put. 
In  excavating  a  trench  the  shovel  straddles  the  trench  and  runs  on 
tractors,  wheels,  or  rollers  on  either  side  of  it.     The  shovel  cuts 
the  trench  ahead  of  it.     As  a  result  it  is  difficult  to  set  sheeting 
and  bracing  close  to  the  end  of  the  trench  while  the  shovel  is 
operating.     Steam  shovels  are  therefore  not  suitable  for  excava- 
tion in  unstable  material,  unless  the  sheeting  is  driven  ahead  of 
the  excavation.     It  is  only  in  the  softest  ground  that  ordinary 
wood  sheeting  can  be  driven  ahead  of  the  excavation.     Steel 
sheet  piling  is  more  suitable  for  such  use.     Fig.  93  1  shows  a  shovel 
at  work  on  a  trench  in  Evanston,  Illinois. 

Shovels  are  equipped  with  extra  long  dipper  handles  to  adapt 
them  to  trench  excavation.  The  dipper  handle  in  the  picture  is 
longer  than  the  standard  for  this  type  of  machine.  The  method 
of  supporting  the  shovel  can  be  seen  in  the  picture  under  the 
machine  and  the  method  of  bracing  and  of  finishing  the  trench 
by  hand  work  are  also  shown.  The  excavated  material  is  taken 
out  in  the  shovel  and  dropped  on  the  bank  or  into  wagons. 

The  limiting  depth  to  which  trenches  can  be  excavated  by 
steam  shovels  is  about  20  to  25  feet,  where  the  trench  is  too  nar- 
row for  the  shovel  to  enter.  Wider  trenches  are  cut  in  steps  of 
about  15  feet,  the  shovel  working  in  the  trench  for  additional 
depths.  Shovels  are  now  made  to  cut  trenches  as  narrow  as  a 
man  can  enter  to  lay  pipe.  The  greatest  width  that  can  be  cut 
from  one  position  of  the  shovel  is  from  15  to  40  feet,  dependent  on 
the  size  of  the  shovel.  Occasionally  a  combination  of  a  drag  line 
1  Courtesy,  Sanitary  District  of  Chicago. 


STEAM   SHOVELS 


253 


and  a  steam  shovel  can  be  used,  as  on  the  construction  of  the 
Calumet  sewer  in  Chicago.     On  this  work  the  first  step  was  cut 
by  a  steam  shovel.     It  was  followed  by  a  drag  line  resting  on  the 
step  thus  prepared,  and  excavating  the  remaining  distance  to 
grade.     The  depth  of 
the  trench  in  this  work 
averaged  about  25  to 
30  feet. 

Steam  shovels  are 
rated  according  to 
their  tonnage  and  the 
capacity  of  the  dipper 
in  cubic  yards.  Both 
are  necessary  as  the 
size  of  the  dipper  is 
varied  for  the  same 
weight  of  machine, 
dependent  on  the  char- 
acter of  the  material 
being  excavated.  For 
rock  the  dipper  is 
made  smaller  than 
for  sand.  Gillette  in 
his  Hand  Book  of 
Cost  Data  gives  the 
coal  and  water  con- 
sumption of  steam 
shovels  as  shown  in 
Table  52.  The  per- 
formance of  ^^  FiG^^i^mShov^t  Work  cm  Sewer  Trench 
shovels  is  recorded  for  North  Shore  Intercepting  Sewer,  Evanston, 
in  Table  53.  The  Illinois, 
conditions  of  the 

work  have  a  marked  effect  on  the  output  of  the  shovel.  A 
shovel  in  a  thorough  cut,  i.e.,  in  a  trench  just  wide  enough 
for  the  shovel  to  turn  180  degrees  but  too  narrow  to  run 
cars  or  wagons  along  side  of  it,  will  perform  less  than  one- 
half  of  the  work  that  it  can  perform  in  a  side  cut,  i.e.,  where 
the  cars  can  be  run  along  side  the  shovel  which  turns  less  than 
90  degrees. 


254 


CONSTRUCTION 


TABLE  52 

COAL  AND  WATER  CONSUMPTION  BY  STEAM  SHOVELS 
(From  Handbook  of  Cost  Data,  by  H.  P.  Gillette) 


Weight  in  tons  

35 

45 

55 

65 

75 

90 

Dipper,  cubic  yards  
Coal,  tons  per  10  hour  day  

U 

3 

1 

H 

11 

2 

U 

2 

3 
2\ 

Water,  gallons  per  10  hour  day.  . 

1500 

2000 

2500 

3000 

4000 

4500 

TABLE  53 

PERFORMANCE  BY  STEAM  SHOVELS 


Cost  in 

Weight 

Dipper 

Depth 

Width 

10-Hour 

Cents, 

in 

Cubic 

of  Cut, 

of  Cut 

Perform- 

per 

Authority 

Remarks 

Tons 

Yards 

Feet 

ance 

Cubic 

Yard 

25 

1 

9 

36  in. 

85 

22.6 

R.  T.   Dana  Eng.   Rec., 

1 

69:581 

25 

1 

8 

35  in. 

96 

23.5 

do. 

2 

70 

2 

26 

16  ft. 

569 

6.7 

do. 

3 

30 

1 

15-18 

60  in. 

300 

A.  B.  McDaniel  Excavat- 

4 

ing    Machinery 

15 

! 

14 

134  ft. 

400 



Eng.  Cont'r,  8-25-09 

5 

8 

36 

/Very 
\wide 

16  yd.  } 
cars     / 

Marion  Steam  Shovel  Co. 

6 

55 

296 

H.  P.  Gillette's  Cost  Data 

7 

65 

2| 

280 

do. 

Greater 
than 
78  in. 

\  700 

30.  6  | 

G.  C.   D.    Lenth,    Eng. 
News-Record,  85:22 

8 

Remarks: 

1.  One  runner  at  $5.00,  one  fireman  at  $2.31,  two  laborers  at  $1.70  each,  supplies  at  $4.50, 

and  interest  and  depreciation  on  200  days  per  year,  $4.00.     Total  per  day,  $19.21. 
Material,  clay  and  gravel. 

2.  Average  of  11  jobs  with  the  same  shovel. 

3.  Cost  per  day,  one  runner  at   $5.00,   one  craneman  at  $3.60,   one  fireman  at  $2.00, 

7  roller  men  at  $1.50  each,  supplies  $9.00  and  interest  and  depreciation  on  $9000 
at  200  days  per  year  $8.00.     Total,  $38.10. 

4.  Hard  clay. 

5.  Stiff  clay  for  the  basement  of  a  building  in  Chicago. 

6.  Stripping  ore.     This  is  a  maximum  record.     The  average  was  about  three  hundred 

and  twenty  16  cubic  yard  cars  per  day. 

7.  Blasted  mica  schist. 

8.  General  average. 


DRAG  LINE  AND  BUCKET  EXCAVATORS      255 

134.  Drag  Line  and  Bucket  Excavators. — A  drag  line  exca- 
vator is  shown  in  Fig.  94.  The  back  of  the  bucket  is  attached  to 
a  drum  on  the  engine  by  means  of  a  cable  passing  over  the  wheel 
in  the  end  of  the  long  boom.  The  front  of  the  bucket  is  attached 
by  another  cable  directly  to  another  drum  on  the  engine.  In 
operation  the  bucket  is  raised  by  its  rear  end  and  dropped  out  to 
the  extremity  of  the  boom.  It  is  then  dragged  over  the  ground 
towards  the  machine,  digging  itself  in  at  the  same  time.  When 
filled  the  bucket  is  raised  by  tightening  up  on  the  two  cables, 
swung  to  one  side  by  means  of  the  movable  boom,  and  dumped. 


FIG.  94.— Drag  Line  at  Work  on  Trench  for  Drain  Tile. 

Drag  line  excavators  will  perform  as  much  work  as  steam 
shovels  under  favorable  conditions.  They  are  less  expensive  in 
first  cost  and  operation,  and  are  equally  reliable  but  they  are  not 
adapted  to  the  more  difficult  situations  where  steam  shovels  can 
be  used  to  advantage.  Drag  lines  are  suitable  only  for  relatively 
wide  trenches  in  material  requiring  no  bracing,  and  in  a  locality 
where  relatively  long  stretches  of  trench  can  be  opened  at  one 
time. 

The  bucket  excavator  differs  from  the  drag  line  in  that  the 
bucket  can  be  lifted  vertically  only  and  the  types  of  buckets  used 
in  the  two  types  of  machine  are  different.  The  bucket  may  be 
self  filling  of  the  orange-peel  or  clam-shell  type,  or  a  cylindrical 
container  which  must  be  filled  by  hand.  A  drag  line  can  be 


256  CONSTRUCTION 

easily  converted  into  a  boom  and  bucket  excavator.  Boom  and 
bucket  excavators  are  well  adapted  to  use  in  deep,  closely  braced 
trenches  and  shafts. 

135.  Excavation  in  Quicksand.1 — A  sand  or  other  granular 
material  in  which  there  is  sufficient  upward  flow  of  ground  water 
to  lift  it,  is  known  as  quicksand.     Its  most  important  property, 
from  the  viewpoint  of  sewer  construction,  is  its  inability  to  sup- 
port any  weight  unless  the  sand  is  so  confined  as  to  prevent  flow- 
ing of  the  sand,  or  unless  the  water  is  removed  from  the  sand. 

Excavation  in  quicksand  is  troublesome  and  expensive  and  is 
frequently  dangerous.  The  material  will  flow  sluggishly  as  a 
liquid,  it  cannot  be  pumped  easily,  and  its  excavation  causes  the 
sides  of  the  trench  to  fall  in  or  the  bottom  to  rise.  The  founda- 
tions of  nearby  structures  may  be  undermined,  causing  collapse 
and  serious  damage.  These  conditions  may  arise  even  after  the 
backfilling  has  been  placed  unless  proper  care  has  been  taken. 
The  greatest  safeguard  against  such  dangers  is  not  only  to  exer- 
cise care  in  the  backfilling  to  see  that  it  is  compactly  tamped  and 
placed,  but  to  leave  all  sheeting  in  position  after  the  completion  of 
the  work. 

The  ordinary  method  of  combating  quicksand  and  in  conduct- 
ing work  in  wet  trenches  is  to  drive  water-tight  sheeting  2  or  3  feet 
below  the  bottom  of  the  trench,  and  to  dewater  the  sand  by  pump- 
ing. When  dry  it  can  be  excavated  relatively  easily.  A  more 
primitive  but  equally  successful  method  is  to  throw  straw,  brick- 
bats, ashes,  or  other  filling  material  into  the  trench  in  order  to 
hold  the  excavation  once  made,  or  this  may  supplement  the 
attempts  at  pumping,  or  the  wet  sand  may  be  bailed  out  in  buckets. 
Successful  excavation  in  quicksand  requires  experience,  resource- 
fulness, and  a  careful  watch  for  unexpected  developments.  The 
well  points  described  in  Art.  142  are  used  for  dewatering  quick- 
sand. 

136.  Pumping  and  Drainage. — Ground  water  is  to  be  expected 
in  nearly  all  sewer  construction  and  provision  should  be  made  for 
its  care.     Where  geological  conditions  are  well  known  or  where 
previous  excavations  have  been  made  and  it  is  known  that  no 
ground  water  exists  it  may  be  safe  to  make  no  provision  for 
encountering  ground  water.     Where  ground  water  is  to  be  expected 

1  See  article  by  J.  R.  Gow,  Journal  New  England  Waterworks  Ass'n, 
Sept.,  1920,  also  Public  Works,  Vol.  50,  p.  98. 


TRENCH  PUMP 


257 


the  amount  must  remain  uncertain  within  certain  rather  wide 
limits  until  actually  encountered. 

In  order  to  avoid  the  necessity  for  pumping,  or  working  in  wet 
trenches  it  is  sometimes  possible  to  build  the  sewer  from  the  low 
end  upwards  and  to  drain  the  trench  into  the  new  sewer.  The 
wettest  trenches  are  the  most  difficult  to  drain  in  this  manner  as 
the  material  is  usually  soft  and  the  water  so  laden  with  sediment 
as  to  threaten  the  clogging  of  the  sewer.  It  is  undesirable  to  run 
water  through  the  pipes  until  the  cement  in  the  joints  has  set. 
This  necessitates  damming  up  the  trench  for  a  period  which  may 
be  so  long  as  to  flood  the  trench  or  delay  the  progress  of  the  work. 
If  it  is  not  possible  to  drain  the  tvench  through  the  sewer  already 
constructed  the  amount  of  water  to  be  pumped  can  be  reduced 
by  the  use  of  tight  sheeting. 

Pumps  for  dewatering  trenches  must  be  proof  against  injury 
by  sand,  mud,  and  other  solids  in  the  water.  For  this  purpose 
pumps  with  wide  passages  and  without  valves  or  ^ 

packed  joints  are  desirable.  The  types  of  pumps 
used  are:  simple  flap  valve  pumps  improvised  on 
the  job,  diaphragm  pumps,  jet  pumps,  steam 
vacuum  pumps,  centrifugal  pumps,  and  recipro- 
cating pumps.  All  are  of  the  simplest  of  their 
type  and  little  attention  is  paid  to  the  economy  of 
operation  because  of  the  temporary  nature  of  their 
service. 

137.  Trench  Pump. — A  simple  pump  which  can 
be  improvised  on  the  job  is  shown  in    section  in 
Fig.    95.      Its  capacity,  is    about    20   gallons   per 
minute  but  its  operation  is  backaching  work.     It  is 
inexpensive,  quickly  put  together  and  may  be  a 
help  in  an  emergency.     It  is  to  be  noted  that  the 
passages  are  large  and  straight,  that  there  are  no        FIQ  Q5 
packed  joints,  and  that  the  velocity  of  flow  is  so      improvised 
small  that  it  is  not  liable  to  clogging  by  picking  up  Trench  Pump, 
small  objects. 

138.  Diaphragm  Pump. — The  type  of  pump  shown  in  Fig.  96 
is  the  most  common  in  use  for  draining  small  quantities  of  water 
from  excavations.     It  is  known  as  the  diaphragm  pump  from  the 
large  rubber  diaphragm  on  which  the  operation  depends.     The 
pump  is  made  of  a  short  cast-iron  cylinder,  divided  by  the  rubber 


258 


CONSTRUCTION 


diaphragm  or  disk  to  the  center  of  which  the  handle  is  connected. 
The  valve  is  shown  at  the  center  of  the  disk.  As  the  diaphragm 
is  lifted  the  valve  remains  closed,  creating  a  partial  vacuum  in  the 

suction  pipe  and  at  the 

same  time  discharging 
the  water  which  passed 
j  through  the  valve  on  the 
previous  down  stroke. 
When  the  valve  is 
lowered  the  foot  valve 
on  the  suction  pipe 
closes,  holding  the  water 
in  place,  and  the  valve 
in  the  pump  opens 
allowing  the  water  to 
flow  out  on  top  of  the 
disk  to  be  discharged 
on  the  next  up  stroke. 
Table  54  shows  the 
capacities  of  some  dia- 
phragm pumps  as  rated 
by  the  manufacturers.  The  smaller  sizes  are  the  more  frequently 
used  and  are  equipped  with  a  3-inch  suction  hose  with  strainer  and 
foot  valve.  They  are  not  adapted  to  suction  lifts  over  10  to  12 
feet.  Where  greater  lifts  are  necessary  one  pump  may  discharge 
into  a  tub  in  which  the  foot  valve  of  a  higher  pump  is  submerged. 

TABLE  54 
CAPACITIES  OF  DIAPHRAGM  PUMPS 


FIG.  96. — Diaphragm  Pump. 
Courtesy,  Edson  Manufacturing  Co. 


Diameter  of 

Diameter  of 

Length  of 

Capacity  per 

Cylinder, 

Suction, 

Stroke  in 

Stroke, 

Inches 

Inches 

Inches 

Gallons 

6 

3 

4 

0.49 

8£ 

4 

6 

1.47 

9* 

24 

0.75 

12t* 

3 

1  25 

12|* 

Power  driven  by  1 

horse-power  engine 

0.58f 

*  Diameter  of  diaphragm, 
t  Gallons  per  minute 


JET  PUMP 


259 


139.  Jet  Pump. — The  simplicity  of  the  parts  of  the  jet  pump 
is  shown  in  Fig.  97.  It  has  a  distinct  advantage  over  pumps 
containing  valves  and  moving 
parts  in  that  there  are  no  obstruc- 
tions offered  to  the  passage  of 
solids  as  well  as  liquids  through 
the  pump.  It  is  not  economical 
in  the  use  of  steam,  however.  It 
operates  by  means  of  a  steam  jet 
entering  a  pipe  at  high  velocity 
through  a  nozzle.  This  action 
causes  a  vacuum  which  will  lift 
water  from  6  to  10  feet.  The 
lower  the  suction  lift,  however, 
the  greater  the  efficiency  of  the 
work.  The  sizes  and  capacities 
of  jet  pumps  as  manufactured  by 
the  J.  H.  McGowan  Co.  are  shown 
in  Table  55, 


FIG.  97.— McGowan  Steam  Jet 

Pump. 
Courtesy,  The  John  H.  McGowan  Co. 


TABLE  55 

CAPACITIES  OF  JET  PUMPS 
(J.  H.  McGowan  Co.) 


Size  of  Pump 

Capacity, 

Approximate 

and 

Discharge  Pipe, 

Steam  Pipe, 

Gallons 

Horse-power 

Suction  Pipe, 

per 

Required 

Inches 

Inches 

Inches 

Minute 

I 

\ 

1 

8 

2 

1 

1 

1 

15 

3 

H 

1 

i 

20 

4 

H 

u 

I 

30 

6 

2 

n 

! 

40 

8 

2£ 

2 

i 

50 

10 

3 

2^ 

i 

60 

15 

4 

3^ 

H 

85 

25 

140.  Steam  Vacuum  Pumps. — This  type  of  pump  depends  on 
the  condensation  of  steam  in  a  closed  chamber  to  create  a  vacuum 
which  lifts  water  into  the  chamber  previously  occupied  by  the 


260 


CONSTRUCTION 


steam  and  from  which  the  water  is  ejected  by  the  admission  of 
more  steam.  The  best  known  pumps  of  this  type  are  the  Pulsom- 
eter,  manufactured  by  the  Pulsometer  Steam  Pump  Co.,  the 
Emerson,  manufactured  by  the  Emerson  Pump  and  Valve  Co.; 
and  the  Nye  Pump,  manufactured  by  the  Nye  Steam  Pump  and 
Machinery  Co. 

A  section  of  a  Pulsometer  is  shown  in  Fig.  98.     It  consists  of 
two  bottle-shaped  chambers  A  and  B  with  their  necks  communi- 


K-2 


Sect! 
Discharge 


FIG.  98 — Pulsometer  Steam  Vaccum  Pump. 

eating  at  the  top  and  each  opening  into  the  outlet  chamber  0 
through  a  check  valve.  Steam  is  admitted  at  the  top  and  enters 
chamber  A  or  B  according  to  the  position  of  the  steam  valve  C 
as  shown.  This  steam  valve  is  a  ball  which  is  free  to  roll  either 
to  the  right  or  left  and  forms  a  steam-tight  joint  with  whichever 
seat  it  rests  upon.  In  normal  operation  chamber  A  would  be 
filled  with  water  as  the  steam  enters  the  cylinder.  At  the  same 
time  a  check  valve  at  the  top  opens  to  admit  a  small  quantity  of 
air  which  forms  a  cushion  insulating  the  steam  from  the  water, 
reduces  the  condensation  of  the  steam,  and  serves  as  a  cushion 


STEAM   VACUUM   PUMPS 


261 


for  the  incoming  water  on  the  opposite  stroke.  The  pressure  of 
the  steam  depresses  the  surface  of  the  water  without  agitation 
and  forces  the  water  through  the  check  valve  F  into  the  discharge 
chamber  0.  When  the  water  falls  to  the  level  of  the  discharge 
chamber  the  even  surface  is  broken  up  and  the  intimate  contact 
of  the  steam  and  water  condenses  the  former  instantaneously. 
This  forms  a  vacuum  in  chamber  A  which,  assisted  by  a  slight 
upward  pressure  in  chamber  B 
caused  by  the  incoming  water, 
immediately  pulls  the  ball  C 
over  to  the  other  seat  and 
directs  the  steam  into  chamber 
B.  The  vacuum  in  chamber  A 
now  draws  up  a  new  .charge  of 
water  through  the  suction  pipe 
into  the  chamber. 

A  section  of  the  Emerson 
pump  is  shown  in  Fig.  99.  The 
pump  consists  of  two  vertical 
cylinders  B  and  C.  Each 
chamber  has  a  suction  valve 
L  at  the  bottom,  opening  up- 
ward from  a  common  chamber 
from  which  the  discharge  pipe 
U  extends.  On  the  top  of 
each  chamber  is  a  baffle  plate 
G  which  operates  to  distribute 
the  steam  evenly  to  the  two 
chambers  and  to  prevent  it 
from  agitating  the  surface  of 
the  water  in  the  chambers. 
A  condenser  nozzle  F  is  con- 
nected with  the  bottom  of  the  opposite  chamber  by  a  pipe 
into  which  a  check  valve  opens  upward.  As  the  pressure  in 
the  chamber  alternates  water  will  be  injected  through  F  into  the 
opposite  chamber  and  condense  the  steam  therein,  promptly 
forming  a  vacuum.  An  air  valve  P  admits  a  small  quantity  of 
air  while  the  chamber  is  filling  with  water,  the  air  acting  as  an 
insulating  cushion  as  in  the  Pulsometer.  Valve  0,  just  above  the 
top  connection  S  is  used  to  regulate  the  amount  of  steam  that 


FIG.  99. — Emerson  Steam  Vaccum 
Pump. 


262  CONSTRUCTION 

enters  the  pump.  The  top  connection  S  has  two  ports,  one  leading 
to  each  chamber.  An  oscillating  valve  enclosed  in  it  admits  the 
steam  through  these  ports  to  the  two  chambers  alternately.  This 
valve  is  driven  by  a  small  three-cylinder  engine,  the  crank  shaft  of 
which  extends  into  the  top  connection  in  the  center  of  the  bearing 
on  which  the  valve  oscillates.  A  positive  geared  connection  is 
made  between  the  valve  and  the  engine  and  so  arranged  that  the 
engine  will  run  faster  than  the  valve. 

The  action  of  these  pumps  consists  of  alternately  filling  and 
emptying  the  two  chambers.  They  will  continue  operation  with- 
out attention  or  lubrication  so  long  as  the  steam  is  turned  on.  In 
view  of  the  simplicity  of  their  operation  and  make-up,  their  ability 
to  handle  liquids  heavily  charged  with  solids,  and  their  reasonable 
steam  consumption  these  pumps  are  widely  used  for  pumping 
water  in  construction  work.  They  have  an  added  advantage 
that  no  foundation  or  setting  is  required  for  them  as  they  can  be 
hung  by  a  chain  from  any  available  support. 

These  pumps  are  manufactured  in  sizes  varying  from  25  to 
2500  gallons  per  minute  at  a  25-foot  head,  and  with  a  steam  con- 
sumption of  about  150  pounds  per  horse-power  hour.  They 
reduce  about  4  per  cent  in  capacity  for  each  10  feet  of  additional 
lift.  They  will  operate  satisfactorily  between  heads  of  5  to  150 
feet,  with  a  suction  lift  not  to  exceed  15  feet.  Lower  suction  lifts 
are  desirable  and  the  best  operation  is  obtained  when  the  pump  is 
partly  submerged.  The  steam  pressure  should  be  balanced  against 
the  total  head.  It  varies  from  50  to  75  pounds  for  lifts  up  to 
50  feet,  and  increases  proportionally  for  higher  lifts.  The  dryer 
the  steam  the  lower  the  necessary  boiler  pressure. 

141.  Centrifugal  and  Reciprocating  Pumps. — The  details  of 
these  pumps,  their  adaptability  to  various  conditions,  and  their 
capacities  are  given  in  Chapter  VII.     The  centrifugal  is  better 
adapted  to  trench  pumping  as  it  is  not  so  affected  by  water  con- 
taining sand  and  grit,  but  for  clear  water,  high  suction  lifts  and 
fairly  permanent  installations,  reciprocating  pumps  can  be  used 
with  satisfaction. 

142.  Well  Points. — In  dewatering  quicksand  a  method  fre- 
quently attended  with  success  is  to  drive  a  number  of  well  points 
into  the  sand  and  connect  them  all  to  a  single  pump.     Figure  100 
shows  a  well  point  system  used  on  sewer  work  in  Indiana.     The 
well  points  are  3  feet  apart  and  are  connected  to  a  2J-inch  header 


ROCK  EXCAVATION  263 

which  in  turn  is  connected  to  six  Nye  pumps,  each  with  a  capacity 
of  200  gallons  per  minute  for  a  lift  of  50  feet.  The  number  and 
size  of  well  points  and  pumps  to  use  will  depend  on  conditions  as 
met  on  the  job.  On  a  piece  of  work  in  Atlantic  City  1  the  equip- 
ment consisted  of  two  complete  outfits  each  comprising  one  hundred 
IJ-inch  by  36-inch  No.  60  well  points,  one  hundred  6-foot  lengths 
of  rubber  hose,  about  600  feet  of  suction  main,  one  hundred 
valved  T  connections,  and  a  7  X  8-inch  Gould  Triplex  Pump  with 
a  capacity  of  200  gallons  per  minute,  belted  to  a  7|  horse-power 
motor. 


FIG.  100.— Well  Points  Pumped  by  Nye  Steam  Vacuum  Pump. 

143.  Rock  Excavation. — A  common  definition  of  rock  used  in 
specifications  is:  whenever  the  word  Rock  is  used  as  the  name  of 
an  excavated  material  it  shall  mean  the  ledge  material  removed 
or  to  be  removed  properly  by  channeling,  wedging,  barring,  or 
blasting;  boulders  having  a  volume  of  9  (this  volume  may  be 
varied)  cubic  feet  or  more,  and  any  excavated  masonry.  No  soft 
disintegrated  rock  which  can  be  removed  with  a  pick,  nor  loose 
shale,  nor  previously  blasted  material,  nor  material  which  may 
have  fallen  into  the  trench  will  be  measured  or  allowed  as  rock. 

Channeling  consists  in  cutting  long  narrow  channels  in  the 
rock  to  free  the  sides  of  large  blocks  of  stone.  The  block  is  then 
loosened  by  driving  in  wedges  or  it  is  pried  loose  with  bars.  It  is 
a  method  used  more  frequently  in  quarrying  than  in  trench  exca- 
.  News,  Vol.  75,  1916  p.  1050. 


264 


CONSTRUCTION 


vation  where  it  is  not  necessary  to  preserve  the  stone  intact.  In 
blasting,  a  hole  is  drilled  in  the  rock,  and  is  loaded  with  an  explosive 
which  when  fired  shatters  the  rock  and  loosens  it  from  its  position. 
In  drilling  rock  by  hand  the  drill  is  manipulated  by  one  man 
who  holds  it  and  turns  it  in  the  hole  with  one  hand  while  striking 
it  with  a  hammer  weighing  about  4  pounds  held  in  the  other  hand, 
or  one  man  may  hold  and  turn  the  drill  while  one  or  two  others 
strike  it  with  heavier  hammers.  In  churn  drilling  a  heavy  drill 

is  raised  and  dropped  in  the  hole, 
the  force  of  the  blow  developing 
from  the  weight  of  the  falling 
drill.  Hand  drills  are  steel  bars 
of  a  length  suitable  for  the  depth 
of  the  hole,  with  the  cutting  edge 
widened  and  sharpened  to  an 
angle  as  sharp  as  can  be  used 
without  breaking.  The  drill  bar 
is  usually  about  |th  of  an  inch 
smaller  than  the  diameter  of  the 
face  of  the  drill. 

Wedges  used  are  called  plugs 
and  feathers.  They  are  shown 
in  Fig.  101  which  shows  also  the 
method  of  their  use .  The  feathers 
are  wedges  with  one  round  and 
one  flat  face  on  which  the  flat 
faces  of  the  plug  slide. 

144.  Power  Drilling. — In  power  drilling  the  drill  is  driven  by  a 
reciprocating  machine  which  either  strikes  and  turns  the  drill  in 
the  hole,  or  lifts  and  turns  it  as  in  churn  drilling,  or  the  drill  may 
be  driven  by  a  rotary  machine  which  is  revolved  by  compressed 
air,  steam,  or  electricity.  There  are  many  different  types  of 
machines  suitable  for  drilling  in  the  different  classes  of  material 
encountered  and  for  utilizing  the  various  forms  of  power  available. 
A.  jack  hammer  drill  is  shown  in  Fig.  102.  In  its  lightest 
form  the  drill  weighs  about  20  pounds  and  is  capable  of  drilling 
f-inch  holes  to  a  depth  of  4  feet.  Heavier  machines  are  available 
for  drilling  larger  and  deeper  holes.  The  same  machine  can  be 
adapted  to  the  use  of  steam  or  compressed  air.  When  in  use  the 
point  of  the  drill  is  placed  against  the  rock  and  a  pressure  on  the 


FIG.  101. — Plug  and  Feathers  for 
Splitting  Rock. 


POWER  DRILLING 


265 


handle  opens  a  valve  admitting  air  or  steam.  The  piston  is  caused 
to  reciprocate  in  the  cylinder,  striking  the  head  of  the  drill  at 
each  stroke.  The  drill  is  revolved  in  the  hole  by  hand  or  by  a 
mechanism  in  the  machine.  A  hollow  drill  can  be  used  by  means 
of  which  the  operator  admits  air  or  steam  to  the  hole,  thus  blow- 
ing it  out  and  keeping  it  clean.  These  machines  have  the  advan- 
tage of  small  size,  portability  and  simplicity.  They  can  be  easily 
and  quickly  set  up  and  the  drills  can  be  changed  rapidly.  Their 
undesirable  features  are  the  vibration  transmitted  to  the  operator 
and  the  dust  raised  in  the  trench. 


i  St'd  Drill 
Throttle- 


FlG.  102. — Jack  Hammer  Rock  Drill. 


FIG.  103.— Tripod  Drill, 


A  type  of  drill  heavier  and  larger  than  the  jack  hammer  drill 
is  shown  in  Fig.  103.  It  requires  some  form  of  support  such  as  a 
tripod,  or  in  tunnel  work  it  can  be  braced  against  the  roof  or  sides. 
Some  data  on  steam  and  air  drills  are  given  in  Table  56.  The 
effect  of  the  length  of  the  transmission  pipe,  temperature  of  the 
outside  air,  pressure  at  the  boiler  or  compressor,  etc.,  will  have  a 
marked  effect  on  the  amount  of  steam  or  air  to  be  delivered  to  the 
drill.  Compressed  air  is  affected  more  than  steam  by  these  out- 
side factors,  but  it  has  an  advantage  in  that  as  it  loses  in  pressure 
it  increases  in  volume  so  that  the  loss  of  power  is  not  so  marked. 
Gillette  states: 


266  CONSTRUCTION 

We  may  assume  that  a  cubic  foot  of  steam  will  do 
practically  the  same  work  in  a  drill  as  a  cubic  foot  of  com- 
pressed air  at  the  same  pressure,  because  neither  the  steam 
nor  the  air  acts  expansively  to  any  great  extent  in  a  drill 
cylinder,  due  to  the  late  cut-off.  This  being  so  ...  one 
pound  of  steam  is  equivalent  to  nearly  30  cubic  feet 'of 
free  air  ...  all  at  the  same  pressure  of  75  pounds  per 
square  inch.  If  a  drill  consumes  at  the  rate  of  100  cubic 
feet  of  free  air  per  minute  ...  it  would  therefore  consume 
240  pounds  of  steam  (at  75  pounds  pressure)  per  hour. 
.  .  .  Where  not  more  than  three  or  four  drills  are  to  be 
operated,  probably  no  power  can  equal  compressed  air 
generated  by  gasoline.  It  will  require  12  horse-power  to 
compress  air  for  each  drill,  hence  1J  gallons  of  gasoline  will 
be  required  per  hour  per  drill  while  actually  drilling. 

TABLE  56 

DATA  ON  ROCK  DRILLS 
(From  H.  P.  Gillette) 


1 

Diameter  of  cylinder  in  inches  

21 

2^ 

21 

3i 

31 

31 

Length  of  stroke  in  inches  

5 

6 

61 

6f 

6f 

71 

Length  of  drill  from  end  of  crank  to 

end  of  piston 

36 

43 

50 

50 

50 

52 

Depth  of  hole  drilled  without  change 

of  bit,  inches 

15 

20 

24 

24 

24 

24 

Diameter  of  supply  inlet.     Standard 

pipe,  inches 

3 

3 

i 

1 

1 

Approximate  strokes  per  minute  with 

• 

4 

60  pound  pressure  at  the  drill  

500 

450 

375 

350 

325 

300 

Depth  of  vertical  hole  each  machine 

will  drill  easily,  feet  

6 

8 

10 

14 

16 

20 

Diameter  of  holes  drilled,  inches  

j  to  1^  as  desired 

Diameter  of  octagon  steel,  inches.  .  .  . 

I  to  | 

I  to  1 

1  to  1| 

U  to  IJ 

1|  to  11 

Htoll 

Best  size  of  boiler  to  give  plenty  of 

steam  at  high  pressure,  horse-power 

0 

8 

8 

9 

10 

12 

Best   size    of   supply   pipe   to    carry 

steam  100  to  200  feet,  inches  

1 

i 

3 

1 

1 

U 

Weight    of    drill    unmounted,    with 

wrenches  and  fittings,  not  boxed, 

pounds  .  . 

128 

190 

265 

315 

385 

390 

Weight  of  tripod,   without   weights, 

not  boxed,  pounds  

80 

160 

160 

160 

210 

275 

Weight    of    holding    down    weights, 

not  boxed,  pounds  

120 

270 

270 

285 

330 

375 

Cubic    feet   of   free   air    per    minute 

required  to  run  one  drill  at   100 

pounds 

92 

104 

126 

146 

154 

160 

For  more  than  one  drill,  multiply  the  value  in  the  above  line  by  the  following  factors: 
For  2  drills,  1.8;  5  by  4.1;  10  by  7.1;  15  by  9.5;  20  by  11.7;  30  by  15.8;  40  by  21.4;  70  by 
33.2. 


POWER  DRILLING  267 

/ 

Since  gasoline  air-compressors  are  self  regulating,  when  the 
drill  is  not  using  air  very  little  gasoline  is  burned  by  the 
gasoline  engine  driving  the  compressor.  A  gasoline  com- 
pressor possesses  other  very  important  economic  advan- 
tages over  a  small  steam-driven  plant.  First,  there  is  the 
saving  in  wages  of  firemen  and  second,  there  is  the  saving  in 
hauling  and  pumping  of  water  and  the  hauling  of  fuel. 
The  cost  of  gasoline  is  often  less  than  the  cost  of  coal  for 
operating  a  small  plant. 

An  electric  drill1  operated  on  the  principle  of  the  solenoid 
does  away  with  motor,  valves,  pipes,  vapor,  freezing,  and  other 
difficulties  attendant  on  the  use  of  steam  or  air. 

The  rates  of  drilling  in  different  classes  of  rock  are  shown  in 
Table  57.  Frequent  changes  of  drills  and  relocation  of  tripods 
will  materially  reduce  the  performance  of  a  drill,  for  as  much  as 
45  minutes  may  be  lost  in  making  a  new  set  up.  In  this  the  jack 
hammer  drills  show  their  advantage  as  no  time  is  lost  in  a  set  up. 

TABLE  57 
RATES  OF  ROCK  DRILLING 

Rates  in  Feet  per  Ten-hour  Shift.    Vertical  Holes  10-20  Feet  Deep. 
(From  Gillette) 

Hard  Adirondack  granite 48 

Maine  and  Massachusetts  granite 45-50 

Mica-schist  of  New  York  City.     Possible 60-70 

Mica-schist  of  New  York  City.     Average 40-50 

Hard,  Hudson  River  trap  rock 40 

Soft  red  sand  stone  of  Northern  New  Jersey 90 

Hard  limestone  near  Rochester,  N.  Y 70 

Limestone  of  Chicago  Drainage  Canal 70-80 

Douglass,  Indiana,  syenite.     Difficult  set  ups 36 

Canadian  granite  on  Grand  Trunk  R.  R 30 

Windmill  point,  Ontario  limestone: 

3f-inch  drills 75 

2f-inch  drills 60 

2Hnch  drills 37 

145.  Steam  or  Air  for  Power. — The  choice  between  steam  or 
air  is  dependent  on  the  conditions  of  the  work.     Steam  is  unde- 
sirable in  tunnels  on  account  of  the  heat  produced.     In  open  cut 
1  Mun.  Engineering,  Vol.  53,  p.  6. 


268 


CONSTRUCTION 


work  it  is  at  a  disadvantage  because  of  the  loss  of  power  due  to 
radiation  from  the  hose  or  pipe.  The  life  of  the  hose  is  not  so 
long  as  when  air  is  used,  escaping  steam  causes  clouds  of  vapor 
which  obscure  the  work,  and  serious  burns  may  occur  due  to  hot 
water  thrown  from  the  exhaust.  It  is  advantageous  since  leaks 
may  be  easily  discovered  and  remedied,  it  requires  less  machinery 
than  air,  and  it  is  sometimes  less  expensive.  With  compressed 
air,  gasoline  or  electric  motors  can  be  used  for  operating  the  com- 
pressors. 

TABLE  58 
ROCK  BLASTING 

(From  Gillette) 


Depth 

Distance 

Distance 

Character  of  Material 

Powder  Used  per  Hole 

of 
Hole, 

Back  of 
Face, 

Hole  to 
Hole, 

Feet 

Feet 

Feet 

Limestone  of  Chicago 

Drainage  Canal  

40  per  cent  dynamite 

12 

8 

8 

Sandstone                     \ 

200  pounds 

1  20 

18 

14 

black  powder 

/ 

Granite                         s 

2  pounds 

J12 

41 

4|  to  5 

60  per  cent  dynamite 

/ 

Pit  Mining,  Treadwell, 

Mine,  Alaska  

12 

2* 

6 

146.  Depth  of  Drill  Hole.— The  depth  of  the  hole  is  dependent 
on  the  character  of  the  work.  The  deepest  holes  can  be  used  in 
open  cut  work  where  the  shattered  rock  is  to  be  removed  by  steam 
shovel.  The  face  can  be  made  10  to  15  feet  high.  The  depth  of 
the  hole  in  center  cut  tunnel  facings  are  from  6  to  10  or  even  12 
feet.  In  the  bench  the  depth  is  equal  to  the  height  of  the  bench. 
In  narrow  trenches  where  the  rock  is  to  be  removed  by  derrick  or 
thrown  into  a  bucket  by  hand,  the  hole  should  be  sufficiently  deep 
to  shatter  the  rock  to  a  depth  of  at  least  6  inches  below  the 
finished  sewer.  Frequently  shooting  to  this  depth  at  one  shot 
cannot  be  done  due  to  the  built  up  condition  of  the  neighborhood 
or  other  local  factors.  The  depth  of  the  hole  in  trench  work 
should  not  much  exceed  the  distance  between  holes.  Deep  holes 


SPACING  OF  DRILL  HOLES  269 

are  usually  desirable  as  a  matter  of  economy  in  saving  frequent 
set  ups,  but  the  holes  cannot  be  made  much  over  20  feet  in  depth 
without  increasing  the  friction  on  the  drill  to  a  prohibitive  amount. 

147.  Diameter  of  Drill  Hole. — The  diameter  of  the  hole  should 
be  such  as  to  take  the  desired  size  of  explosive  cartridge.     The 
common  sizes  of  dynamite  cartridges  are  from  }  inch  to  2  inches  in 
diameter.    In  drilling,  the  diameter  of  the  hole  is  reduced  about  one- 
eighth  of  an  inch  at  a  time  as  the  drill  begins  to  stick.     This  reduc- 
tion should  be  allowed  for,  and  experience  is  the  best  guide  for  the 
size  of  the  hole  at  the  start.     In  general  the  softer  or  more  faulty 
or  seamy  the  rock,  the  more  frequent  the  necessary  reductions  in 
size  of  bit.1     For  hard  homogeneous  rock  the  holes  can  be  drilled 
10  feet  or  more  without  changing  the  size  of  the  drill  bit. 

148.  Spacing  of  Drill  Holes. — The  spacing  of  holes  in  open 
cut  excavation  is  commonly  equal  to  the  depth  of  the  hole.     The 
character  of  the  material  being  excavated  has  much  to  do  with  the 
spacing  of  the  holes.     The  spacing,  diameter  and  depth  of  holes 
used  on  some  jobs  is  shown  in  Table  58.     Gillette  states: 

It  is  obviously  impossible  to  lay  down  any  hard  and 
fast  rule  for  drill  holes.  In  stratified  rock  that  is  friable, 
and  in  traps  that  are  full  of  natural  joints  and  seams,  it  is 
often  possible  to  space  the  holes  a  distance  apart  somewhat 
greater  than  their  depth,  and  still  break  the  rock  to  com- 
paratively small  sizes  upon  blasting.  In  tough  granite, 
gneiss,  syenite,  and  in  trap  where  joints  are  few  and  far 
between,  the  holes  may  have  to  be  spaced  3  to  8  feet  apart 
regardless  of  their  depth  for  with  wider  spacing  the  blocks 
thrown  down  will  be  too  large  to  handle  with  ordinary 
appliances.  Since  in  shallow  excavations  the  holes  can 
seldom  be  much  further  apart  than  one  to  one  and  one-half 
times  their  depth  we  see  that  the  cost  of  drilling  per  cubic 
yard  increases  very  rapidly  the  shallower  the  excavation. 
Furthermore  the  cost  of  drilling  a  foot  of  hole  is  much 
increased  where  frequent  shifting  of  the  drill  tripod  is 
necessary. 

The  common  practice  in  placing  drill  holes  is  to  put 
down  holes  in  pairs,  one  hole  on  each  side  of  the  proposed 
trench;  and  if  the  trench  is  wide  one  or  more  holes  are 
drilled  between  these  two  side  holes  2  but  in  narrow  trench 

lYor  types  of  drill  bits  see  article  by  T.  H.  Proske,  Mining  and  Scientific 
Press,  March  5,  1910. 

2  These  intermediate  holes  are  seldom  more  than  3  feet  apart. 


270  CONSTRUCTION 

work,  such  as  for  a  12-inch  pipe,  one  hole  in  the  middle  of 
the  trench  will  usually  prove  sufficient. 

The  holes  are  spaced  about  3  feet  apart  longitudinally.  After 
the  holes  have  been  completed  they  should  be  plugged  to  keep 
out  dirt  and  water. 

SHEETING  AND  BRACING 

149.  Purposes  and  Types. — Sheeting  and  bracing  are  used  in 
trenching  to  prevent  caving  of  the  banks  and  to  prevent  or  retard 
the  entrance  of  ground  water.     The  different  methods  of  placing 
wooden  sheeting  are  called  stay  bracing,  skeleton  sheeting,  poling 
boards,  box  sheeting,  and  vertical  sheeting.     Steel  sheeting  is 
usually  driven  to  secure  water  tightness  and  if  braced  the  bracing 
is  similar  to  the  form  used  for  vertical  wooden  sheeting. 

150.  Stay  Bracing. — This  consists  of  boards  placed  vertically 
against  the  sides  of  the  trench  and  held  in  position  by  cross 
braces  which  are  wedged  in  place.     The  purpose  of  the  board 
against  the  side  of  the  trench  is  to  prevent  the  cross  brace  from 
sinking  into  the  earth.     The  boards  should  be  from  1JX4  inches 
to  2X6  inches  and  3  to  4  feet  long.     The  cross  braces  should  not 
be  less  than  2X4  inches  for  the  narrowest  trenches   and  larger 
sizes  should  be  used  for  wider  trenches.     The  spacing  between  the 
cross  braces  is  dependent  on  the  character  of  the  trench  and  the 
judgment  of  the  foreman.     Stay  bracing  is  used  as  a  precautionary 
measure  in  relatively  shallow  trenches  with  sides  of  stiff  clay  or 
other  cohesive  material.     It  should  not  be  used  where  a  tendency 
towards   caving  is  pronounced.     Stay  bracing  is  dangerous  in 
trenches  where  sliding  has  commenced  as  it  gives  a  false  sense  of 
security.     The  boards  and  cross  braces  are  placed  in  position 
after  the  trench  has  been  excavated. 

151.  Skeleton  Sheeting. — This  consists  of  rangers  and  braces 
with  a  piece  of  vertical  sheeting  behind  each  brace.     A  section  of 
skeleton  sheeting  is  shown  in  Fig.  104  with  the  names  of  the  differ- 
ent pieces  marked  on  them.     This  form  of  sheeting  is  used  in 
uncertain  soils  which  apparently  require  only  slight  support,  but 
may  show  a  tendency  to  cave  with  but  little  warning.     When  the 
warning  is  given  vertical  sheeting  can  be  quickly  driven  behind 
the  rangers  and  additional  braces  placed  if  necessary.     The  sizes 
of  pieces,  spacing  and  method  of  placing  should  be  the  same  as 


POLING  BOARDS 


271 


for  complete  vertical  sheeting  in  order  that  this  may  be  placed  if 
necessary. 

152.  Poling  Boards. — These  are  planks  placed  vertically 
against  the  sides  of  the  trench  and  held  in  place  by  rangers  and 
braces.  They  differ  from  vertical  sheeting  in  that  the  poling 
board  is  about  3  or  4  feet  long.  It  is  placed  after  the  trench  has 
been  excavated ;  not  driven  down  with  the  excavation  like  vertical 
sheeting.  An  arrangement  of  poling  boards  is  shown  in  Fig.  105. 
This  type  of  support  is  used  in  material  that  will  stand  unsup- 
ported for  from  3  to  4  feet  in  height.  Its  advantages  lie  in  that  no 
driving  is  necessary,  thus  saving  the  trench  from  jarring;  no 


FIG.  104.— Skeleton  Sheeting. 


FIG.  105.— Poling  Boards. 

Showing  Different  Types  of  Cross  Bracing. 


sheeting  is  sticking  above  the  sides  of  the  trench  to  interfere  with 
the  excavation;  and  only  short  planks  are  necessary. 

The  method  of  placing  poling  boards  is  as  follows:  Excavate 
the  trench  as  far  as  the  cohesion  of  the  bank  will  permit.  Poling 
boards,  1J  inch  to  2  inch  planks,  6  inches  or  more  in  width,  are 
then  stood  on  end  at  the  desired  intervals  along  each  side  of  the 
trench  for  the  length  of  one  ranger.  The  poling  boards  may  be 
held  in  place  by  one  or  two  rangers.  Two  are  safer  than  one  but 
may  not  always  be  necessary.  If  one  ranger  is  to  be  used  it  is 
placed  at  the  center  of  the  poling  board.  After  the  poling  boards 
are  in  position  the  rangers  are  laid  in  the  trench  and  the  cross 


272 


CONSTRUCTION 


braces  are  cut  to  fit.  If  wedges  are  to  be  used  for  tightening  the 
cross  braces,  the  cross  braces  are  cut  about  2  inches  short.  If 
jacks  are  to  be  used  the  braces  are  cut  short  enough  to  accommo- 
date the  jacks  when  closed,  or  adjustable  trench  braces  may  be  used 
as  shown  in  Fig.  106.  The  use  of  extension  braces  saves  the  labor 
of  fitting  wooden  braces.  With  everything  in  readiness  in  the 
trench,  the  cross  brace  is  pressed  against  the  ranger  which  is  thus 
held  in  place.  The  wedge  or  jack  is  then  tightened  holding  the 
poling  boards  and  cross  brace  in  position. 

153.  Box  Sheeting. — Box  sheeting  is  composed  of  horizontal 
planks  held  in  position  against  the  sides  of  the  trench  by  vertical 
pieces  supported  by  braces  extending  across  the  trench.     The 

arrangement  of  planks  and  braces 
for  box  sheeting  is  shown  in  Fig. 
106.  This  type  of  sheeting  is 
used  in  material  not  sufficiently 
cohesive  to  permit  the  use  of 
poling  boards,  and  under  such 
conditions  that  it  is  inadvisable 
to  use  vertical  sheeting  which 
protrudes  above  the  sides  of  the 
trench  while  being  driven.  This 
sheeting  is  put  in  position  as  the 
trench  is  excavated.  No  more 
of  the  excavation  than  the  width 
of  three  or  four  planks  need  be 
FIG.  106.— Box  Sheeting.  unsupported  at  any  one  time.  In 

Showing  Different  Types  of  Cross  Bracing,    placing    the    sheeting    the    trench 

is  excavated  for  a  depth  of  12  to 

24  inches.  Three  or  four  planks  are  then  placed  against  the  sides 
of  the  trench  and  are  caught  in  position  by  a  vertical  brace  which 
is  in  turn  supported  by  a  horizontal  cross  brace. 

154.  Vertical  Sheeting. — This  is  the  most  complete  and  the 
strongest  of  the  methods  for  sheeting  a  trench.     It  consists  of  a 
system  of  rangers  and  cross  braces  so  arranged  as  to  support  a 
solid  wall  of  vertical  planks  against  the  sides  of  the  trench.     An 
arrangement  of  complete  vertical  sheeting  i?  shown  in  Fig.  107. 
This  type  can  be  made  nearly  water  tight  by  the  use  of  matched 
boards,  Wakefield  piling,  steel  piling,  etc.    Wakefield  piling  is 
made  up  of  three  planks  of  the  same  width  and  usually  the  same 


VERTICAL  SHEETING 


273 


.  107.—  Vertical  Sheeting. 


thickness.    They  are  nailed  together  so  that  the  two  outside  planks 

protrude  beyond  the  inside  one  on  one  side,  and  the  inside  one 

protrudes  beyond  the  two  out- 

side ones  on  the  other  side  as 

shown  in  Fig.  108.     The  pro- 

truding  inside    plank  forms  a 

tongue  which  fits  into  the  groove 

formed  by  the  protruding  out- 

side planks  of  the  adjacent  pile. 
In  placing  vertical  sheeting 

the  trench  is  excavated  as  far 

as  it  is  safe  below  the  surface. 

Blocks  of  the  same  thickness 

as  the  sheeting  are  then  placed 

against  the  bank  at  the  middle 

and  at  the  ends  of  two  rangers 

on  opposite  sides  of  the  trench. 

The  ranger  rest  against  blocks, 

and  are  held  away  from  the  sides  of  the  trench  by  them.    Cross 

braces  are  next  tightened  into  position  opposite  the  blocks  to 
hold  the  rangers  in  place.  After  the 
skeleton  sheeting  is  in  place  the  planks 
forming  the  vertical  sheeting  are  put  in 

FIG.  108.-Wakefield  Sheet  Position  with  a  chisel  edge  cut  on  the 

Piling.  lower  end  of  the  plank,  with  the  flat  side 

against  the  bank.     The  planks  should  be 

driven  with  a  maul,  the  edge  of  the  plank  following  closely  behind 
the  excavation.     In  relatively  dry  work  the  driving  of  the  plank 
is  facilitated  by  excavating  beneath  the  edge  as 
it  is  driven.     The  upper  end  of  the  sheeting  should 
be  protected  by  a  malleable  steel  or  iron  cap  to 
prevent  brooming  of  the  lumber.     A  cap  is  shown 
in  Fig.  109.     A  sledge  hammer  may  be  used  for 
driving  when  the  lumber   is    protected.     If  the 
sheeting  is  to  start  at  the  surface  and  is  to  be 
j  driven  by  hand,  the  first  length  should  not  exceed 
4  feet  unless  a  platform  is  erected  for  the  driver. 
Succeeding  lengths  may  be  longer,  the  driver  stand- 
ing on  planks  supported  on  the  cross  braces  in  the  trench.    Steam 
hammers  and  pile  drivers  are  sometimes  used  for  driving  sheeting. 


FIG.  109. 

Section  through 

Malleable  Steel 

Driving  Cap. 


274  CONSTRUCTION 

The  framework  of  the  sheeting  should  be  placed  with  a  cross 
brace  for  each  end  of  each  ranger  and  a  cross  brace  for  the  middle 
of  each  ranger.  If  the  ends  of  two  rangers  rest  on  the  same  cross 
brace  an  accident  displacing  one  ranger  will  be  passed  on  to  the 
next  and  might  cause  a  progressive  collapse  of  a  length  of  trench, 
whereas  the  movement  of  an  independently  supported  ranger 
should  have  no  effect  on  another  ranger.  The  cross  braces 
should  have  horizontal  cleats  nailed  on  top  of  them  as  shown  in 
Fig.  107  to  prevent  the  braces  from  being  knocked  out  of  place  by 
falling  objects.  In  driving  vertical  sheeting  a  vacant  place  will 
be  left  behind  each  cross  brace  corresponding  to  the  original  block 
placed  to  hold  the  ranger  away  from  the  bank.  This  is  an  unde- 
sirable, feature  in  the  use  of  vertical  sheeting.  It  is  ordinarily 
remedied  by  slipping  in  planks  the  width  of  the  slot  and  wedging 
or  nailing  them  against  the  convenient  cross  bracing.  In  extremely 
wet  trenches,  after  all  other  pieces  of  vertical  sheeting  are  in  place, 
the  original  cleat  behind  the  cross  brace  can  be  knocked  out  and  a 
piece  of  sheeting  slipped  into  this  opening  and  driven.  Care 
must  be  taken  in  this  event  not  to  drive  the  rangers  down  when 
driving  the  sheeting.  If  the  bracing  begins  to  drop,  it  should  be 
supported  by  vertical  pieces  between  the  rangers  and  resting  on  a 
sill  at  the  bottom  of  the  trench. 

155.  Pulling  Wood  Sheeting. — Wood  sheeting  is  pulled  after 
the  completion  of  the  trench  by  a  device  shown  in  Fig.  110.  In 
wet  trenches  where  the  removal  of  the  sheeting 
would  permit  a  movement  of  the  banks,  result- 
ing in  danger  to  the  sewer  or  other  structures, 
the  sheeting  should  be  left  in  place  in  the  trench. 
If  sufficient  saving  can  be  made  the  sheeting  is 
cut  off  in  the  trench  immediately  above  the  danger 
line,  usually  the  ground  water  line.  The  cutting 

.  is  done  with  an  axe  or  by  a  power  driven  saw 
FIG.  110.  —  Steel    ,     .      ,  »       , 
Clamp  for  Pull-  devised  for  the  purpose. 

ing  Wood  Sheet-        156.  Earth  Pressures.1 — The  various  theories 

ing.  of    earth    pressure    are  so    conflicting    in    their 

conclusions     as     to     be    confusing.     Rankine's 

theory,  the  most  frequently  used,  assumes    that    the   pressure 

increases    with    the    depth,    whereas    Meem's     theory 2    leads 

1  Earth  Pressures,  Old  Theories  and  New  Test  Results,  Eng.  News-Record, 
Vol.  85,  1920,  p.  632. 

2  Trans.  Am.  Society  Civil  Eng'rs,  Vol.  60,  1908. 


EARTH  PRESSURES  275 

to  an  opposite  conclusion.  The  discussion  following  Meem's 
article  is  very  illuminating.  It  indicates  that  no  matter  how 
good  the  theory,  practical  experience  together  with  the  use  of 
generous  sizes  and  close  spacing  are  the  best  guides  for  bracing 
trenches  and  coffer  dams.  All  are  not  possessed  with  the  desired 
practical  experience  and  some  basis  on  which  to  commence  work 
is  essential.  Another  factor  affecting  computations  of  sizes 
based  on  theory  is  the  tendency  in  practice  to  use  the  same  size 
material  for  rangers  and  braces  on  any  one  job  for  all  except  very 
deep  trenches  and  other  special  cases.  Occasionally  where  there 
is  an  independent  brace  for  each  end  of  each  ranger,  the  brace  is 
made  thinner,  but  is  of  the  same  depth  as  the  ranger. 

The  application  of  Rankine's  theory  of  earth  pressure  to  the 
computation  of  the  sizes  of  rangers  and  braces  will  be  shown. 
His  formula  for  the  active  earth  pressure  against  a  retaining 
wall  is: 


D       ,         .  cos  6  -  Vcos2  0  -  cos2 
P=whcos0 


cos  0+Vcos2  6—  cos2  0 

in  which  w  =  the  weight  of  earth  in  pounds  per  cubic  foot; 

h  =  depth  in  feet  at  point  at  which  pressure  is  to  be 

determined  ; 
0  =  the  angle  of  surcharge,  or  the  angle  which  the  surface 

makes  with  the  horizontal; 
<£  =  the  angle  of  repose  of  the  earth.     Usually  taken  as 

33°-41'=l£  horizontal  to  1  vertical; 
P  =  the  intensity  of  pressure  in  pounds  per  square  foot  on 

a  vertical  plane  in  a  direction  parallel  to  the  surface 

of  the  ground. 

In  studying  the  pressures  for  trenches  the  surface  of  the  ground 
will  be  assumed  as  horizontal  and  the  formula  reduces  to 


157.  Design  of  Sheeting  and  Bracing.—  The  trench  shown  in 
Fig.  Ill  is  assumed  to  be  constructed  in  moist  sand  weighing  110 
pounds  per  cubic  foot,  with  an  angle  of  repose  of  30  degrees.  The 
material  used  for  sheeting  and  bracing  is  yellow  pine.  The  steps 


276 


CONSTRUCTION 


taken  in  the  design  of  the  sheeting  and  bracing  for  this  trench  are, 
as  follows: 

1.  Earth  Pressure. — Substituting  the  units  given  in  the  data, 
in  Rankine's  formula  for  earth  pressures, 

P  =  36.7/1. 

Because  the  earth  has  been  freshly  cut  and  will  not  be  kept  open 
long  enough  to  break  up  the  cohesiveness  of  the  banks  it  is  cus- 
tomary to  reduce  the  assumed 
pressure  by  dividing  by  2,  3,  or 
4,  according  to  the  natural 
cohesiveness  of  the  material. 
The  cohesiveness  of  sand  is  not 
great,  therefore  the  pressure 
will  be  assumed  as  one-half  of 
the  amount  given  by  the  form- 
ula, or 


yi  "?• 


I 
m 


4"x6 


6x8-~. 


4"x6"      6**V 
[s 6'-8-— ~\ 


\ 6.V— >| 


4"*8' 


5 


w 

4t4lbs/sq.f& 


4BI. 

Diagram  '(% 
of  Pressures! 
on  Sheeting.l 


do. 


!M 


1  ,.      f 

|«-4-VH 


4"x8' 


KIO, 


8x10* 


"FTF 


2.  Thickness  of  Sheeting  and 
Spacing  of  Rangers. — It  is  desir- 
able to  use  the  same  thickness 
of  sheeting  throughout  the  depth 
of  the  trench.  Computations 
should  therefore  be  commenced 
at  the  bottom  of  the  trench 
where  the  pressures  are  the 
greatest  and  the  thickest 
sheeting  will  be  required.  It 
is  necessary  to  determine  by 
trial  a  spacing  for  the  rangers 
and  a  thickness  of  sheeting 
so  that  the  sheeting  is  stressed 
to  its  full  working  strength. 

Having  determined  the  thickness  of  the  sheeting  at  the  bottom, 

the  remainder  of  the  computations  consists  in  determining  the 

spacing  of  the  rangers. 

In  the  example  the  lower  ranger  will  be  assumed  as  3  feet  from 

the  bottom  of  the  trench  and  the  distance  to  the  next  ranger  as 

4  feet. 


FIG.  111. — Diagram  for  the  Design  of 
Wood  Sheeting. 


DESIGN  OF  SHEETING  AND  BRACING 


277 


The  intensity  of  pressure  at  22  feet  9  inches  is  409.5 

pounds  per  square  foot. 
The  intensity  of  pressure  at  26  feet  9  inches  is  481.5 

pounds  per  square  fot. 

The  distribution  of  pressures  is  shown  by  the  diagram  on  Fig.  111. 
The  maximum  bending  moment  is  slightly  below  the  point  mid- 
way between  the  rangers  and  for  a  12-inch  strip  is  10,500  inch 
pounds. 

Assuming  3  inch  sheeting  the  maximum  fiber  stress  is: 

Me     10,400X1.5X12 
/=-=-=  --  --  =  568  pounds  per  square  inch. 

1  \.a/\&( 


The  working  strength  of  yellow  pine  as  given  in  Table  59,  is 
1200  pounds  per  square  inch.  Thinner  sheeting  should  therefore 
be  used. 

TABLE  59 
WORKING  UNIT  STRESSES  FOR  TIMBER 

The  most  used  value  in  the  Building  Codes  of  Baltimore,  Boston,  Cin- 
cinnati, Chicago,  District  of  Columbia,  and  New  York  City 


Wood 

Tension, 
lb.sq.in. 

Com- 
pression 
With 
Grain, 
Ib.sq.  in. 

Com- 
pression 
Across 
Grain, 
Ib.sq.  in. 

Trans- 
verse 
Bending, 
Ib.sq.  in. 

Shear 
With 
Grain, 
Ib.sq.  in. 

Shear 
Across 
Grain, 
Ib.sq.  in. 

Yellow  pine 

1200 

1000 

600 

1200 

70 

500 

White  pine  

800 

800 

400 

800 

40 

250 

Spruce  and  Va.  pine. 
Oak 

800 
1000 

800 
900 

400 

800 

800 
1000 

50 
100 

320 
600 

Hemlock  

600 

500 

500 

600 

40 

275 

Chestnut       .... 

600 

500 

1000 

800 

150 

Locust 

1200 

1000 

1200 

100 

720 

As  published  in  American  Civil  Engineers  Pocket  Book. 

Assuming  2-inch  sheeting,  the  fiber  stress  is  1,300  pounds  per 
square  inch.  This  stress  is  too  large.  By  reducing  the  ranger 
spacing  slightly  the  stress  can  be  brought  within  the  required 
limits. 

Assuming  a  ranger  spacing  of  3  feet  9  inches  the  depth  to  the 
upper  ranger  is  changed  to  23  feet  and  the  maximum  stress  in  the 


278  CONSTRUCTION 

2-inch  sheeting  becomes  1,140  pounds  per  square  inch,  a  satis- 
factory result.  The  results  for  the  computations  for  the  other 
ranger  spacings  are  shown  in  Table  60.  The  spacing  of  the 
rangers  at  the  sheeting  junctions  is  controlled  by  convenience 
and  is  not  computed  so  long  as  it  is  obviously  safe. 

3.  Size  of  Rangers.  —  The  rangers  will  be  assumed  as  16  feet 
long  with  two  end  cross  braces  and  one  intermediate  cross  brace 
for  each  ranger.  Starting  as  before  at  the  bottom  of  the  trench. 

The  area  of  the  panel  below  the  ranger  and  between 
cross  braces  is  24  square  feet. 

The  average  intensity  of  pressure  is  28.25X18  =  508.5 
pounds  per  square  inch. 

The  load  transmitted  to  the  ranger  is  6,000  pounds. 

Similarly  the  load  transmitted  to  the  ranger  from  the 
panel  above  is  6,890  pounds. 

The  total  distributed  load  on  the  ranger  is  12,890  pounds. 

If  b  is  the  vertical  dimension  of  the  ranger  and  d  is  the  hori- 
zontal dimension  in  inches,  then  from  the  beam  theory,  using  /  as 

1,200  pounds  per  square  inch,  ^2  =  o'  *n  w^c^  M  is  expressed 


in  inch  pounds.    The  maximum  bending  moment  is 

Wl    12,200X8X12 

-5-  =  -  5  -  =  155,000  inch-pounds. 
o  o 

Therefore,  bd2  =  775. 

An  8X10  inch  beam  will  fulfill  the  conditions  closely.     Substi- 
tuting these  dimensions  in  the  beam  formula 

155,000X5X12 


~  /  8X1000 

=  1,160  pounds  per  square  inch  tension  in  outer  fiber.     The  results 
of  the  computations  for  other  rangers  are  shown  in  Table  60. 

4.  Size  of  Cross  Braces.  —  The  cross  braces  act  as  columns. 
The  dimensions  of  the  cross  braces  are  determined  by  trial  in  such 
a  manner  that  the  vertical  dimension  of  the  brace  is  equal  to  the 
vertical  dimension  of  the  ranger  and  the  compressive  stress  in 
pounds  per  square  inch  is  computed  from  the  expression, 


Adopted  by  the  Am.  Ry.  and  Maintenance  of  Way  Ass'n  in  1907. 


DESIGN  OF  SHEETING  AND  BRACING 


279 


TABLE  60 

COMPUTATIONS  FOR  SHEETING  AND  BRACING  FOR  TRENCH  SHOWN  IN 

FIG.  Ill 

Material  is  moist  sand  weighing  1 10  pounds  per  cubic  foot,  with  an  angle  of  repose  of  30°. 
Lumber  is  yellow  pine,  with  working  stresses  as  given  in  Table  59.     Working  stresses  for 

columns  given  as  S 


ha- 


Sheeting  2  Inches  X  12  Inches 

Cross  Braces 

Maxi- 

Allow- 

Maxi- 

mum 

Actual 
In- 

able 

Depth 

mum 
Bending 
Moment, 

Fiber 
Stress, 
Pounds 

Depth  and 
Description 

Total 
Load, 
Pounds 

Size, 
Inches 

tensity, 
Pounds 
per 

In- 
tensity, 
Pounds 

Inch- 
Pounds 

per 
Square 

Square 
Inch 

per 
Square 

Inch 

Inch 

23'-26  .  75' 

9100 

1140 

end  at  26'  9" 

6,445 

4X8 

202 

784 

19'-23' 

8800 

1100 

int.  at  26'  9" 

12,890 

4X8 

403 

784 

13'-17.5' 

8550 

1070 

end  at  23'  0" 

6,393 

4X8 

200 

784 

8'-13' 

7160 

900 

int.  at  23'  0" 

12,785 

4X8 

400 

784 

(X-6' 

3000 

375 

end  at  19'  0" 

3,930 

4X8 

123 

784 

int.  at  19'  0" 

7,860 

4X8 

246 

784 

end  at  17'  6" 

3,566 

4X8 

112 

684 

int.  at  17'  6" 

7,132 

4X8 

224 

684 

end  at  13'  0" 

4,385 

4X8 

137 

684 

int.  at  13'  0" 

8,770 

4X8 

274 

684 

end  at     8'  0" 

2,270 

4X6 

95 

687 

int.  at     8'  0" 

4,540 

4X6 

189 

667 

end  at     6'  0" 

1,344 

4X6 

56 

584 

int.  at     6'  0" 

2,687 

4X6 

112 

584 

end  at    O7  0" 

432 

4X6 

18 

584 

int.  at     0'  0" 

863 

4X6 

36 

584 

l 

Rangers 


Maxi- 

Area 

In- 

mum 

Maxi- 

Depth 

of 
Panel 
Below 
this 

tensity 
of 
Pressure, 
Pounds 

Total 
Load 
in 

Load  Transmitted  to 
the  Ranger  from  the 

Size, 
Inches 

Bending 
Moment 
in 
Thou- 

mum 
Stress 
Pounds 
per 

Depth, 
Square 

per 
Square 

Pounds 

Panel 

Panel 

Both 

sand 
Inch- 

Square 
Inch 

Feet 

Inch 

Below 

Above 

Panels 

Pounds 

26'  9" 

24 

508.5 

12,200 

6000 

6890 

12,890 

8X10 

155 

1160 

23'  0" 

30 

448 

13,440 

6545 

6240 

12,785 

8X10 

153 

1150 

19'  0" 

32 

378 

12,100 

5860 

2000 

7,860 

8X10 

94.3 

708 

17'  6" 

12 

328.5 

3,942 

1942 

5190 

7,132 

8X10 

85.6 

636 

13'  0" 

36 

274.5 

9,880 

4690 

4080 

8,770 

8X10 

105 

790 

8'0" 

40 

189 

7,560 

3480 

1060 

4,540 

6X   8 

54.4 

850 

6'0" 

16 

126 

2,020 

960 

1727 

2,687 

6X   8 

32.2 

503 

(XO" 

48 

54 

2,590 

863 

0 

863    |  6X   8 

10.4 

161 

280  CONSTRUCTION 

in  which  S  =  permissible    crushing    across   the    grain   in   a 

column  whose  length  is  greater  than  15  diam- 
eters; 

Si  =  unit  working  compressive  strength  of  wood ; 
1  =  length  of  the  column; 
d  =  smallest  dimension  of  the  column; 
I  and  d  are  in  the  same  units. 

The  lower  intermediate  cross  brace  supports  a  length  of  8  feet  of 
the  lower  ranger  on  which  the  load  has  been  found  to  be  12,890 
pounds.  The  load  on  the  end  cross  brace  for  the  same  ranger  is 
one-half  of  this  or  6,445  pounds.  The  length  of  each  brace  is 
4  feet  4  inches.  From  Table  59,  Si  is  1,000  pounds  per  square 
inch.  From  the  column  formula,  S  is  784  pounds  per  square 
inch. 

A  4X8  inch  cross  brace  is  the  smallest  that  is  feasible.  This 
is  stressed  only  12,890  pounds  or  403  pounds  per  square  inch, 
which  is  well  within  the  permissible  limits.  The  results  of  the 
other  computations  for  cross  braces  are  shown  in  Table  60. 

158.  Steel  Sheet  Piling. — This  is  coming  into  more  general 
use  with  the  increased  cost  of  lumber  and  better  acquaintance 
with  its  superiority  over  wood  under  many  conditions.  Although 
its  first  cost  is  higher  than  that  of  wood,  the  fact  that  with  proper 
care  it  can  be  used  almost  an  indefinite  number  of  times  renders 
it  economical  to  contractors  who  may  have  an  opportunity  to 
make  repeated  use  of  it.  The  life  of  good  yellow  pine  sheeting 
with  the  best  of  care  may  be  as  much  as  three  or  four  seasons. 
With  no  particular  care  it  will  be  destroyed  at  the  first  using. 
Fig.  112  shows  various  sections  of  steel  piling  used  for  trench 
sheeting.  These  forms  are  practically  water  tight  and  aid  mate- 
rially in  maintaining  dry  trenches.  The  piling  can  be  made  water 
tight  by  slipping  a  piece  of  soft  wood  between  the  steel  sections 
when  they  are  being  driven,  or  by  pouring  in  between  the  piles 
some  dry  material  which  will  swell  when  wet.  The  piling  is  gen- 
erally driven  by  a  steam  hammer  and  is  pulled  by  attaching  a 
ring  through  a  bolt  hole  in  the  pile,  or  by  grasping  the  pile  with  a 
clutch  that  tightens  its  grasp  as  the  pull  increases.  An  inverted 
steam  hammer  attached  to  the  pile  is  sometimes  used  in  pulling 
it.  The  impulses  of  the  hammer  together  with  a  steady  pull  on 
the  cable  serve  to  drag  out  the  most  stubborn  piece  of  piling. 


LOCATING  THE  TRENCH 


281 


LINE  AND  GRADE 

159.  Locating  the  Trench. — In  order  to  locate  a  trench  a  line 
of  stakes  should  be  driven  at  about  50-foot  intervals  along  the 
center  line  of  the  proposed  sewer  before  excavation  is  commenced. 
Reference  stakes  or  reference  points  to  this  line  are  located  at 
some  fixed  offset  or  easily  described  point,  or  the  stakes  marking 


/•~~*~j*r_: 

//&  r 


> 

Section 


15"  Arched- Web  Section  (No.S.P.HI5) 

Modulus,  Single  =  11.80          46.5  lb.  per  sq.ft.  of  Wall  227.02  kg. per sq. meter  of 

,,         .Interlocked.  14.11          68.12  lb.    „  tin.  ft.  of  Bar.  66.49  7,  '„    ifn.   „         „  Bar. 


Section  Modu/> 


14  Arched-  Web  Section  (No.S.RE  14) 

'du/vi,  Single    -  7.61  3S  lb.  per  sq.fr.  of  Wall. 

»       ,  Interlocked'  8.89  40.83/1.  ,,  l,n.  fr.  of  Bar. 


170. 90  kg.  per  so.  meter  of  Wall. 
60.76    »,     n  Im.    »       „  Bar. 


(  No.S.RFIS) 

.  _  lb.  persg.  ft.  of  Wall. 
60  lb    ••  lm.fr.  of  Bar. 


234. 34  leg.  per  sq.  mater  of  Wolf. 
89.29  ,,    „  /in.    „     n  Bar. 


'     |2|'U  "straight-  Web  Section.    "~ 
(Np.S.PAIZ) 


Section  Modulus,  Single  -  S.73 
n          n        '  Interlocked  =6.2S 


FIG.  112.  —  Sections  of  Lackawanna  Steel  Sheet  Piling. 

the  center  line  of  the  trench  may  be  driven  at  some  constant 
offset  distance  one  side  of  the  trench,  in  order  to  avoid  danger  of 
loss  or  disturbance  of  the  stakes.  Grade  or  cut  is  seldom  marked 
on  the  line  of  preliminary  stakes,  although  the  approximate  cut 
may  be  indicated. 

For  hand  excavation  the  foreman  lays  out  the  trench  from 
these  stakes.  In  machine  work  the  operator  guides  the  machine 
so  as  to  follow  the  line  of  the  stakes. 


282 


CONSTRUCTION 


160.  Final  Line  and  Grade. — After  the  excavation  of  the  trench 
has  proceeded  to  within  a  foot  or  two  of  the  final  depth,  the  grade 
and  line  are  transferred  to  markers  supported  over  the  center  of 
the  trench.  The  markers  are  horizontal  boards  spanning  the 
trench  and  held  in  position  either  by  nails  driven  into  stakes  at 
the  side  of  the  trench,  by  nails  driven  into  the  sheeting,  or  by 
weights  holding  the  boards  on  the  ground.  Two  stakes  driven  in 
the  ground  at  the  side  of  the  trench  as  shown  in  Fig.  113  are  the 
common  method  of  support.  If  the  banks  are  too  weak  to  stand 
under  the  jarring  of  the  driving  of  the  stakes,  or  pavement  or 
other  causes  prevent  their  use  the  horizontal  cross  piece  may  be 
weighted  down  by  bricks  or  a  bank  of  earth.  The  cross  pieces 
are  located  about  every  25  feet  along  the  trench  and  at  any  con- 


Center'^  Line-'  Wf- 


FIG.  113.— Methods  for  the  Support  of  the  Grade  Line. 

venient  distance  above  the  surface  of  the  ground.  The  nearer 
the  ground  the  stronger  the  support  but  the  greater  the  inter- 
ference with  work  in  the  trench.  The  center  line  of  the  sewer  is 
marked  on  the  cross  pieces  after  they  are  set,  and  vertical  struts 
are  nailed  on  them  with  one  edge  of  the  strut  straight,  vertical, 
and  on  the  center  line  as  shown  in  Fig.  1.  The  corresponding 
edge  should  be  used  on  all  struts  in  order  to  avoid  confusion. 
The  edge  is  placed  in  a  vertical  position  by  means  of  a  plumb 
bob  or  carpenter's  level. 

The  cut  to  the  invert  of  the  sewer  is  recorded  to  an  even 
number  of  feet  where  practicable  by  driving  a  nail  in  the  upright 
strut  so  that  the  top  edge  of  the  nail  is  at  the  desired  elevation 
above  the  sewer,  or  the  upright  is  nailed  with  its  top  at  the  proper 
number  of  feet  above  the  sewer  invert.  The  cut  is  marked  on 
the  upright  in  feet,  tenths,  and  hundredths  from  the  recorded 
point  to  the  elevation  of  the  invert. 

The  inspector  should  watch  these  grade  markers  with  care  by 
sighting  back  along  them  to  see  that  they  are  in  line  and  have  not 


TRANSFERRING  GRADE  AND  LINE  TO  THE  PIPE      283 


Grade 


Ground  Surface 


Grade 
Rod 


moved.  In  quicksand  or  caving  material  the  marks  may  move 
during  the  setting  of  the  pipes  and  the  levelman  should  be  on  the 
job  constantly. 

When  excavation  is  being  done  by  machine  the  depth  of  the 
excavation  is  controlled  by  the  operator  who  maintains  a  sighting 
rod  on  the  machine  in  line  with  the  grade  marks  on  the  uprights. 

161.  Transferring  Grade  and  Line  to  the  Pipe. — In  transfer- 
ring grade  and  line  to  the  sewer  a  light  strong  string  is  stretched 
tightly  from  nail  to  nail  on  the 

uprights  marking  the  line  and 
grade.  A  rod  with  a  right  angle 
projection  at  the  lower  end,  as 
shown  in  Fig.  114,  is  marked 
with  chalk  or  a  notch  at  such 
a  distance  from  the  end  that 
when  the  mark  Is  held  on  the 
grade  cord  the  lower  portion  of 
the  rod  which  projects  into  the 
pipe  will  rest  on  the  invert.  The 
pipe  is  placed  in  line  by  hang- 
ing a  plumb  bob  so  that  the 
plumb  bob  string  touches  the 
grade  and  center  line  cord.  These* 
marks  are  taken  only  as  fre- 
quently as  may  be  necessary  to 

keep  the  sewer  in  line.  An  experienced  workman  can  maintain 
the  line  by  eye  for  considerable  distances.  Measurements  should 
never  be  taken  to  the  top  of  the  pipe  in  order  to  determine  position 
and  grade  as  the  variations  in  the  diameter  of  the  pipe  may  cause 
appreciable  errors. 

The  position  and  elevation  of  the  forms  for  brick,  concrete, 
and  unit  block  sewers  are  located  by  reference  to  the  grade  line, 
or  they  may  be  placed  under  the  immediate  direction  of  the  survey 
party,  or  by  specially  located  stakes.  For  large  sewers  requiring 
deep  and  wide  excavation  the  grade  and  line  stakes  are  driven  in 
the  bottom  of  the  trench  about  a  foot  above  the  finished  grade. 
This  requires  the  constant  presence  of  an  engineer  who  is  usually 
available  on  work  of  such  magnitude. 

162.  Line  and  Grade  in  Tunnel. — In  tunnels,  line  and  grade 
are  given  by  nails  driven  in  the  roof,  the  progress  of  excavation  or 


FIG.  114. — Diagram  Showing  the  Use 
of  the  Grade  Rod  for  Fixing  the 
Elevation  of  a  Sewer. 


284  CONSTRUCTION 

the  shield  being  followed  by  eye  and  the  forms  set  by  direct 
measurement  to  the  nails. 

TUNNELING 

163.  Depth. — The  depth  at  which  it  becomes  economical  to 
tunnel  depends  mainly  upon  the  character  of  the  material  to  be 
excavated  and  on  the  surface  conditions.     In  soft  dry  material 
with  unobstructed  working  space  at  the  surface,  open  cut  may  be 
desirable  to  depths  as  great  as  35  or  40  feet.     Tunnels  are  cut  in 
rock  at  depths  of  15  feet  or  less.     In  some  very  wet  and  running 
quicksand  encountered  in  the  construction  of  sewers  for  the  Sani- 
tary District  of  Chicago  it  was  found  economical  to  tunnel  at  depths 
of  20  feet  and  less.     Crowded  conditions  on  the  surface,  expensive 
pavements,  or  extensive  underground  structures  near  the  surface 
may  make  it  advantageous  to  tunnel  at  shallower  depths  than 
would  otherwise  be  economical.     Winter  is  the  best  season  for 
tunneling  as  the  workmen  are  protected  from  the  elements  and 
labor  is  more  plentiful. 

164.  Shafts. — In  sinking  a  shaft  in  soft  material,  the  excava- 
tion is  usually  done  by  hand,  the  material  being  thrown  into  a 
bucket  which  is  hoisted  to  the  surface  and  dumped.     The  size  of 
the  shaft  is  independent  of  the  size  of  the  sewer  and  depends  princi- 
pally on  the  machinery  which  it  is  necessary  to  lower  into  the 
tunnel.     Ordinarily  a  shaft  6  feet  in  the  clear  is  satisfactory.     A 
method  of  timbering  a  shaft  is  shown  in  Fig.  115.     Because  of 
the  timbering  the  shaft  must  be  started  sufficiently  large  at  the 
top  to  finish  with  the  desired  dimensions  at  the  bottom.     This 
excess  size  is  sometimes  obviated  by  driving  the  sheeting  at  an 
angle  to  maintain  the  same  size  of  shaft  from  top  to  bottom. 

In  timbering  a  shaft  as  shown  in  Fig.  115  the  upper  frame  is 
staked  securely  in  position  at  the  surface  of  the  ground.  This 
frame  is  composed  of  timbers  fastened  together  in  the  form  of  a 
square  with  the  ends  of  the  timbers  extending  about  12  inches  on 
all  sides.  The  protruding  ends  are  used  to  hold  the  frame  in 
position.  Excavation  is  begun  inside  the  frame,  and  sheeting  is 
driven  around  the  outside  of  it  as  excavation  progresses.  Only 
two  or  three  men  can  work  advantageously  at  one  time  in  these 
small  shafts.  The  second  frame  is  made  up  of  the  same  size  tim- 
bers, but  all  are  cut  off  flush  with  the  outside  of  the  square.  The 


SHAFTS 


285 


]/2"<5ingte  Sheeting 


outside  dimensions  of  this  frame  are  such  as  to  allow  sheeting  to  be 
slipped  in  between  it  and  the  sheeting  already  driven.  The  frame 
is  lowered  into  position  and  supported  from  the  upper  frame  by 
vertical  struts  nailed  to  it.  The  lower  end  of  the  sheeting  already 
driven  is  held  out  from  the  lower  frame  by  blocks  of  the  thickness 
of  the  next  length  of  sheeting.  These  blocks  are  removed  as  the 
next  length  of  sheeting  is  placed 
and  driven.  The  driving  of  the 
sheeting  is  facilitated  by  excavating 
beneath  it  as  it  descends. 

The  sizes  of  sheeting  and  timber- 
ing should  be  computed  on  the  same 
basis  as  that  for  trench  sheeting 
except  that  for  depths  greater  than 
30  to  35  feet  Rankine's  Theory  is 
not  applicable  and  judgment  must 
be  relied  on  for  computing  the  sizes 
for  deep  shafts.  In  stiff  dry  ma- 
terial the  pressures  will  change  very 
little  as  the  depth  increases.  Sheet- 
ing is  needed  in  shaft  excavation 
in  rock  only  to  protect  the  work- 
men from  falling  fragments,  but  in 
sand,  particularly  in  quicksand 
and  in  wet  ground,  the  pressures 
increase  directly  with  the  depth  and 
the  sheeting  should  be  computed 
accordingly.  Care  must  be  taken 
to  prevent  the  formation  of  cavities 
behind  the  sheeting,  to  fill  them  if 
formed,  and  to  see  that  all  pieces 
of  the  sheeting  and  bracing  have  a 

firm  bearing.     It  is  difficult  to  prevent  the  collapse  of  the  shaft 
once  the  movement  of  earth  against  the  sheeting  has  commenced. 

Shafts  are  also  sunk  in  soft  ground  by  constructing  a  concrete 
or  metal  shell  resting  on  a  cutting  shoe  on  the  surface.  The 
material  inside  is  dug  out  and  the  shell  sinks  of  its  own  or  added 
weight.  The  first  section  of  the  shell  may  be  from  5  to  10  feet 
long.  As  this  section  sinks  other  sections  are  added.  This  is 
called  the  caisson  method.  It  is  advantageous  in  wet  ground  and 


Section* 


A-A. 


FIG.  115.— Section  of  Shaft  Tim- 
bering. 

Abbot,  Journal  Western  Society  of 
Engineers,  Vol.  22. 


286  CONSTRUCTION 

when  the  shafts  are  to  be  left  as  a  permanent  manhole.  If  a 
permanent  shaft  is  to  be  left  in  an  excavation  being  braced  with 
wood,  the  permanent  lining  should  follow  within  20  to  30  feet  of 
the  shaft  excavation.  This  is  done  to  avoid  the  difficulty  of 
maintaining  a  great  length  of  temporary  wood  shaft  with  the 
danger  of  collapse,  or  of  blocks  or  other  objects  falling  on  the 
workers  below. 

The  distance  between  shafts  is  controlled  by  the  depth  and 
size  of  the  tunnel,  surface  conditions,  and  the  character  of  the 
material  being  tunneled.  Except  where  surface  conditions  are 
crowded  the  shallower  the  cover  to  the  tunnel  the  more  frequent 
the  shafts.  The  advantage  of  frequent  shafts  lies  in  the  possi- 
bility of  removing  excavated  material  from  the  tunnel  promptly, 
and  in  making  ventilation  of  the  tunnel  easier.  The  saving  made 
by  the  construction  of  numerous  shafts  must  be  balanced  against 
the  extra  cost  of  the  shafts.  For  the  shallowest  tunnels  the 
shafts  are  seldom  placed  closer  than  every  500  feet. 

165.  Timbering. — After  the  shaft  has  been  excavated  to  the 
proper  grade  the  tunnel  is  struck  out  either  by  cutting  through 
the  wooden  sheeting  or  by  removing  portions  of  the  caisson 
lining.  Practically  all  tunnels  except  those  in  solid  rock  must  be 
framed  to  some  extent.  Some  of  the  types  of  frames  used  in 
tunnel  construction  are  shown  in  Fig.  116.  Different  combina- 
tions of  these  may  be  used  in  different  classes  of  materials.  In 
solid  rock  which  remains  firm  on  exposure  no  timbering  is  neces- 
sary. Where  the  roof  only  need  be  supported  and  the  sides  are 
strong  enough  to  be  used  for  support,  a  timber  "  hitch  "  or  frame 
supported  on  the  sides  of  the  tunnel  may  be  used.  This  is  suit- 
able for  loose  rock  roofs  with  solid  rock  sides.  Timbering  such  as 
is  shown  in  the  lower  left-hand  corner  of  Fig.  116  becomes  neces- 
sary in  extremely  soft,  wet,  or  swelling  material,  where  the  bottom 
and  sides  as  well  as  the  roof  tend  to  push  in.  The  remaining 
frame  in  Fig.  116  shows  a  form  frequently  used  and  lying 
between  the  two  extremes  indicated.  In  wet  tunnels  a  channel 
may  be  cut  in  the  bottom  below  the  sill  for  drainage  purposes  as 
shown  in  this  form.  The  needle  beam  method  of  timbering  is 
also  shown  in  Fig.  116.  This  method  of  timbering  is  used  mainly 
near  the  heading  because  of  the  speed  and  ease  with  which  it  can 
be  installed,  but  it  is  undesirable  because  of  the  space  occupied. 

The  distance  between  frames  is  dependent  on  the  size  of  the 


TIMBERING 


287 


tunnel  and  the  character  of  the  material.  It  is  seldom  greater 
than  6  feet  and  the  frames  are  sometimes  placed  touching  each 
other.  The  size  of  the  timbering  is  a  matter  of  experience  and  is 
generally  determined  by  the  judgment  of  the  responsible  person 
in  charge  of  the  construction  as  the  result  of  observation  during 
the  progress  of  the  work. 

The  sheeting  between  frames  is  called  poling  boards,  or  spiling 
or  lagging  according  as  it  is  sharpened  and  driven  ahead  of  the 
excavation  or  placed  after  the  excavation  has  progressed.  The 


Needle 

— 

Beam 

~ 

\ 

.^^^'^^//^'/•"r^.1!;:  • 

Longitudinal   Section* 


p  ^pSPPfP7 

Longitudinal  Section 
Showing  Poling  Boards 

Types  of  Frames    and    Timbering 
for  Tunnels. 


Transverse  Section. 


Tunnel  Bracing  Showing 
Needle  Beam  Support- for  Roof. 


FIG.  116. — Types  of  Frames  and  Timbering  for  Tunnels. 

horizontal  strips  placed  between  the  frames  to  keep  them  apart 
are  called  wales. 

In  cutting  out  from  the  shaft  in  soft  materials  requiring  sup- 
port»  where  the  width  of  the  tunnel  is  the  same  or  smaller  than 
that  of  the  shaft,  a  frame  with  a  maximum  width  four  thicknesses 
of  sheeting  less  than  the  width  of  the  tunnel  is  set  up  against  the 
lining  of  the  shaft.  The  vertical  side  pieces  of  the  tunnel  frame 
rest  on  the  bottom  frame  of  the  shaft  as  a  sill  and  are  securely 
wedged  into  position.  As  the  lining  of  the  shaft  at  the  top  is  cut 
away  the  top  poling  boards  of  the  tunnel  are  slipped  in  between  the 
cap  of  the  first  tunnel  frame  and  the  shaft  frame  immediately  above 


288 


CONSTRUCTION 


it.  The  poling  boards  are  driven  with  an  upward  pitch  so  that 
there  may  be  room  to  slip  the  second  length  of  boards  between  the 
next  tunnel  frame  and  the  first  length  of  boards.  The  placing  of 
the  side  sheeting  follows  in  a  similar  manner.  Excavation  is  then 
started  and  the  poling  boards  driven  to  keep  pace  with  it.  The 
next  frame  is  placed  in  position  and  the  previous  sheeting  or 
boards  wedged  out  a  sufficient  distance  to  allow  the  advance 
lining  to  be  slipped  in  when  the  wedges  are  removed.  Waling 
pieces  are  nailed  firmly  between  the  frames  to  hold  them  in  posi- 
tion. The  various  phases  in  the  driving  of  a  12-foot  sewer  tunnel 
in  Seattle  are  shown  in  Fig.  117. 

In  soft  or  running  material  it  may  be  necessary  to  protect  the 
face  of  the  tunnel  by  horizontal  boards,  called  breast  boards, 


•Jt/2  Segmsnftrf 

Arch  Lagging- —^O\ 


'PhaseV:!  Phase'"    Phase  "'Phase' 
3.        +.  5.  6. 

FIG.  117v — Stages  of  Sewer  Tunneling. 

Eng.  Record,  Vol.  69,  1914,  p.  195. 

wedged  back  to  the  last  frame  placed.  The  excavation  is  per- 
formed by  removing  one  board  at  a  time,  excavating  behind  it 
and  then  replacing  it  in  the  advance  position.  The  advance  is 
made  from  the  top  downwards.  This  represents  the  method 
pursued  in  the  most  difficult  material  where  wooden  sheeting 
without  a  shield  is  used.  The  timbering  during  the  advance  may 
be  modified  in  any  manner  that  the  character  of  the  material  will 
permit.  The  timbering  may  lag  behind  the  excavation  a  dis- 
tance of  two  or  more  frames,  or  it  may  be  omitted  altogether. 
Heavier  timbering  may  be  necessary  in  soft,  slipping  or  shattered 
rock. 

166.  Shields. — Shields  are  used  in  tunneling  in  soft  wet 
material  and  are  particularly  suitable  for  work  under  air  pressure. 
They  are  used  in  rock  tunnels  where  water  is  anticipated  or  air 


SHIELDS 


289 


pressure  is  used.  The  shields  often  save  the  expense  and  diffi- 
culty of  timbering  as  the  masonry  of  the  sewer  follows  closely 
behind  the  shield.  Fig.  118  shows  the  arrangement  for  a  shield 
for  tunneling  in  soft  material  in  the  construction  of  the  Milwaukee 
sewers.  The  shield  has  an  exterior  diameter  of  9  feet  4  inches 


^ 

' 


\v:::7::jrd^^^ 

4    .Cast  Iron  Jack  Seat 
10  I  i 


FIG.  118. — Shield  for  Driving  Milwaukee  Sewer  Tunnel. 

Eng.  News-Record,  Vol.  80,  1918,  p.  669. 

and  an  overall  length  of  9  feet  8J  inches.  The  cutting  edge  sec- 
tion is  20  inches  long.  The  shell  is  made  of  one  inch  plate  to  the 
back  of  the  jack  chambers  and  one-half  inch  plate  in  the  tail. 
The  shield  is  driven  by  ten  60-ton  hydraulic  jacks.  The  jacks 


290  CONSTRUCTION 

are  shown  in  position  in  the  figure.  These  jacks  rest  against  the 
finished  tunnel  lining  and  serve  to  consolidate  it  at  the  same  time 
that  they  push  the  shield  into  the  material  to  be  excavated.  The 
face  of  the  tunnel  is  cut  with  a  pick  and  shovel  while  the  jacks 
are  removed  one  at  a  time  and  a  new  ring  of  lining  is  put  in  place. 
The  lining  may  be  temporary  timbering  to  receive  the  thrust  of 
the  jacks,  but  it  is  usually  desirable  that  the  permanent  lining 
follow  immediately  behind  the  shield.  Since  the  shield  is  larger 
than  the  outside  of  the  lining  the  space  left  by  its  passage  should 
be  grouted  immediately  after  it  has  passed. 


FIG.  119.— Method  of  Drilling  and  Loading  Rock  Tunnel  Face. 

Courtesy,  Aetna  Power  Co. 

167.  Tunnel  Machines. — Tunnel  machines  have  been  used 
successfully  on  sewer  tunnels  in  soft  materials,  but  not  in  rock.1 
The  machines  are  of  different  types,  but  in  general  consist  of  a 
revolving  cutting  head,  equipped  with  knives,  and  driven  by  an 
electric  motor.     The  bearing  on  which  the  shaft  for  the  cutting 
head  rests  is  supported  against  the  sides  of  the  tunnel.     The  muck 
is  carried  away  by  means  of  a  conveyor  and  dumped  into  muck 
cars  without  rehandling.     Rapid  progress  can  be  made  with  these 
machines  in  suitable  conditions. 

168.  Rock  Tunnels. — Tunnels  in  rock  are  advanced  by  drilling 
into  the  face  as  shown  in  diagrammatic  form  in  Fig.  119.    The 

1  Tunneling  Machines  Successful  on  Detroit  Sewers,  Eng.  News-Record, 
Vol.  84,  1920,  p.  329. 


VENTILATION  291 

/ 

holes  near  the  center  are  driven  in  at  an  angle  towards  the  center 
and  to  depths  from  6  to  15  feet.  The  harder  the  rock  the  greater 
the  angle  with  the  tunnel.  This  is  called  the  center  cut.  Other 
holes  are  driven  near  the  outer  edge  of  the  tunnel  and  parallel  to 
its  axis.  When  fired,  the  wedge  of  rock  between  the  center  cut 
holes  is  thrown  back  into  the  tunnel  and  a  delayed  explosion 
then  throws  the  sides  into  the  hole  thus  made.  A  final  delay 
thrusting  shot  throws  the  muck  so  formed  away  from  the  face  of 
the  tunnel.  For  tunnels  up  to  6  or  8  feet  in  height  the  entire  bore 
is  cut  out  in  this  fashion.  For  larger  tunnels,  the  upper  portion 
called  the  heading,  is  taken  out  in  this  way,  and  the  remainder, 
called  the  bench,  is  taken  out  by  drilling  and  blowing  holes  normal 
to  the  axis  of  the  tunnel.  The  amount  of  powder  necessary  in 
the  bench  holes  is  much  less  than  that  required  in  the  heading. 

169.  Ventilation. — No  tunnel  more  than  50  feet  long  should 
be  built  without  ventilation.  A  fair  amount  of  air  for  ordinary 
conditions  is  75  cubic  feet  of  free  air  per  minute  per  person  in  the 
tunnel,  and  double  this  amount  for  each  animal.  Where  explosive 
gases  are  met,  or  under  conditions  where  the  tunnel  is  hot,  five  or 
six  times  as  much  air  may  be  needed  in  order  to  cool  the  tunnel  or 
to  dilute  the  gases.  In  order  that  the  air  may  be  fresh  and  cool 
at  the  face  of  the  tunnel  where  work  is  going  on  it  should  be  con- 
ducted to  the  tunnel  face  in  a  pipe  and  blown  out  into  the  tunnel. 
Immediately  following  a  blast  at  the  face  the  current  should  be 
reversed  so  as  to  draw  the  poisonous  gases  out  of  the  tunnel 
through  the  duct.  The  high  pressure  air  line  leading  to  the  drills 
should  be  opened  at  the  same  time  to  create  a  current  towards  the 
face  in  order  to  accelerate  the  clearing  of  the  air  at  the  heading. 
The  capacity  of  the  air  machines  should  be  sufficient  to  exhaust 
four  times  the  volume  of  the  gases  created  by  the  explosion,  in  15 
minutes.  This  will  ordinarily  call  for  a  capacity  of  about  4,000 
cubic  feet  of  free  air  per  minute.  If  the  same  blower  is  to  be 
used  for  exhausting  the  gases  as  for  ventilation  while  work  is  going 
on,  it  should  have  a  high  overload  capacity  to  care  for  this  situa- 
tion. The  air  line  should  be  arranged  to  allow  for  reversal  of  flow. 

The  diameter  of  the  air  pipe  should  be  determined  by  a  study 
of  the  saving  of  the  cost  and  operation  of  the  air  equipment  com- 
pared to  the  increased  cost  of  a  larger  pipe  line.  Other  factors 
affecting  the  size  of  the  pipe  line  to  be  used  are:  the  available 
space  in  the  tunnel,  the  temporary  character  of  the  installation, 


292  CONSTRUCTION 

the  use  of  the  exhaust  from  high-pressure  air  machines  for  the 
purpose  of  ventilation,  etc.  Cast-iron,  spiral-riveted  galvanized 
sheet  iron,  and  canvas  pipes  have  been  used  for  conducting  low- 
pressure  ventilating  air. 

Ventilation  in  tunnels  working  under  air  pressure  is  supplied 
from  the  compressors,  and  the  ah-  is  delivered  near  the  face  of 
the  heading,  except  that  being  used  in  the  locks.  In  tunnels 
using  air  drills,  the  air  for  the  drills  is  conducted  through  a  sep- 
arate pipe  as  it  is  not  economical  to  compress  the  ventilating  air 
to  the  pressure  necessary  to  operate  the  drills. 

170.  Compressed  Air. — Compressed  air  is  used  in  tunnel  work 
to  prevent  the  entrance  of  water  into  the  tunnel  and  to  keep  the 
work  dry.  The  pressure  of  air  used  is  closely  that  of  the  pressure 
of  the  ground  water  but  in  a  large  tunnel  or  a  tunnel  with  a  weak 
roof  the  pressure  may  be  somewhat  lower  on  account  of  the  danger 
of  blowing  through  the  roof.  It  is  evident  that  the  water  pres- 
sure cannot  be  balanced  at  the  top  and  the  bottom  of  the  tunnel. 
To  balance  it  at  the  bottom  makes  a  blow  out  near  the  top  more 
probable.  To  balance  the  pressure  at  the  top  may  leave  the 
bottom  wet.  Judgment  and  care  must  be  exercised  during  con- 
struction and  if  the  pressure  is  balanced  at  or  near  the  bottom  the 
roof  must  be  carefully  guarded  by  grouting  and  puddling  with 
clay,  or  the  surface,  particularly  if  under  water,  may  be  covered 
with  a  clay  bank.  If  the  cavities  in  the  tunnel  lining  are  large, 
sawdust  can  be  mixed  with  the  grout  to  advantage,  the  mixture 
being  pumped  through  holes  in  the  roof  by  hand  or  power  operated 
force  pumps.  "  Blows  "  must  be  carefully  guarded  against  as 
they  endanger  the  lives  of  the  workmen  and  threaten  the  loss  of 
the  tunnel.  The  pressure  and  volume  of  air  supplied  for  some 
large  subaqueous  tunnels  is  shown  in  Table  61. 

Labor  under  compressed  air  is  arduous  and  dangerous  with 
the  best  of  safeguards.1  Pressure  more  than  about  43  pounds 
per  square  inch  cannot  be  used  and  at  this  high  pressure  men  can- 
not work  more  than  four  hours  at  a  time.  Little  or  no  distress 
is  noted  at  pressures  less  than  15  pounds. 

Entrance  and  exit  to  the  tunnel  are  gained  through  air  locks. 
These  are  sheet-iron  cylinders  concreted  into  the  lining  of  the 
tunnel  or  shaft.  Air-tight  iron  doors  are  provided  at  both  ends, 

1  Rules  on  Compressed-Air  Work  of  N.  Y,  State  Industrial  Commission, 
Eng.  News-Record,  Vol.  85,  1920,  p,  1225, 


COMPRESSED  AIR 


293 


TABLE  61 

VOLUME  AND  PRESSURE  OF  COMPRESSED  AIR  IN  TUNNELS 
(American  Civil  Engineers  Pocket  Book) 


Maxi- 

Maxi- 

Average 

Tunnel 

mum 
Distance 
High 
Water 

Mini- 
mum 
Cover 

mum 
Air 
Pressure, 
Pounds 

Air 
Pressure, 
Pounds 

Conditions  and  Cubic  Feet  of 
Free  Air  per  Minute 

in 

per 

to 
Invert, 

Feet 

per 
Square 

Square 

TnoVi 

Feet 

Inch 

incn 

City  and  South 

34 

42 

15 

in     water     bearing-sand.      1660 

London 

cubic  feet  per  minute  per  face. 

When    grouted    1000    to    1300 

cubic  feet  per  minute  per  face 

Blackball 

80 

5 

37 

35 

10,000  cubic  feet  per  minute  per 

face  in  open  ballast  for  some 

time 

Baker  St.  and 

70 

18 

35 

28 

In  gravel,  3300  cubic  feet  of  air 

Waterloo 

per  minute  per  face.     Parallel 

tunnel  1650  cubic  feet  per  min. 

per  face 

Greenwich 

70 

30 

28 

20 

Average  83.5  per  man  per  minute. 

Never  less  than  66.7 

Battery,  East 

94 

12 

42 

26 

In    sand.      Two    working    faces. 

River,  N.  Y. 

Maximum  32,000 

East  River,  N.  Y.. 

93 

8 

42 

27 

Maximum    for    one    face    25,000 

Penn.  R.R. 

cubic  feet  per   minute  for  24 

hours.     Capacity  of  plant  for 

8  faces,  80,400  cubic  feet    per 

minute 

North  River, 

98 

20 

37 

26 

Maximum  in  gravel  10,000  cubic 

N.  Y.,  Penn. 

feet  per  man  per  hour.    Gener- 

R.R. 

ally  ranged  between  1500  and 

5000 

which  open  inwards  towards  the  tunnel.  On  entering  the  lock 
from  the  outside  the  door  to  the  tunnel  is  found  tightly  closed. 
The  outside  door  is  then  closed  by  hand,  the  air  valve  is  opened 
and  air  is  admitted  to  the  lock  until  the  pressure  on  the  lock  side 
of  the  tunnel  door  equalizes  that  on  the  tunnel  side  and  the  tunnel 
door  is  swung  open  by  hand.  When  the  lock  is  open  to  the 
tunnel  the  pressure  in  the  tunnel  keeps  the  outside  door  closed. 
In  order  to  leave  the  tunnel  the  process  is  reversed.  Materials 


294  CONSTRUCTION 

are  passed  through  the  lock  by  the  lock  tender  or  tenders  who 
pass  through  the  lock  with  the  material  if  the  pressure  is  low,  or 
who  manipulate  the  air  outside  of  the  lock  if  the  pressure  is  high. 
If  pressures  of  30  to  40  pounds  are  being  used,  two  or  even  three 
locks  may  be  necessary. 

EXPLOSIVES  AND  BLASTING1 

171.  Requirements. — The  desirable  features  in  an  explosive 
to  be  used  in  trenching  and  tunneling  in  rock  are:    (1)  stability 
in  make  up  so  as  not  to  deteriorate  in  strength  or  to  become 
dangerous  during  storage,  (2)  imperviousness  to  ordinary  varia- 
tions in  temperature  and  moisture,  (3)  insensibility  to  ordinary 
shocks  received  in  transportation  and  handling,  (4)  not  too  diffi- 
cult of  detonation,  (5)  convenient  form  for  transportation  and 
loading  and  for  making  up  charges  of  different  weights,  (6)  the 
non-formation  of  poisonous  gases  when  fired,    (7)  imperviousness 
to  water  and  usefulness  in  wet  holes,  (8)  power  without  bulk,  etc. 

172.  Types  of  Explosives. — Explosives  which  fill  some  or  all 
these  requirements  can  be  divided  into  two  classes,  deflagrating 
and   detonating.    A    deflagration   is   an   explosion   transmitted 
progressively  from  grain  to  grain.     A  detonation  is  a  sudden  dis- 
ruption caused  by  synchronous  vibrations  of  a  wave-like  char- 
acter.    The    deflagrating    explosives    are    represented    by    gun- 
powders   and    contractors'  powders.     They    must    be    carefully 
tamped  in  the  hole  to  develop  their  full  power  and  they  must  be 
ignited  by  a  fuse  or  flame.     They  are  valueless  in  water  or  moist 
holes.     These  powders  are  used  mainly  for  loosening  frozen  earth, 
soft  sandstone,  cemented  gravels  and  similar  materials  where  a 
thrusting  action  rather  than  a  disruption  is  desired.     The  detonat- 
ing explosives  are  most  commonly  represented  by  the  dynamites. 
These  are  exploded  by  a  shock  usually  caused  by  another  explosive 
which  has  been  ignited  by  a  fuse  or  electric  spark,  and  which  is 
known  as  the   "  detonator."     Detonating  explosives  are  more 
powerful  than  deflagrating  explosives  and  are  used  in  all  but  the 
softest  materials. 

1  Taken  mainly  from  the  Engineer  Field  Manual  of  the  U.  S.  Army; 
Safety  Factors  in  the  Use  of  Explosives  by  W.  O.  Snelling,  Technical  Paper 
No.  18,  U.  S.  Bureau  of  Mines;  and  an  article  in  Eng'g  and  Contracting, 
Vol.  52,  1919,  p.  585. 


TYPES  OF  EXPLOSIVES  295 

Gunpowder. — This  is  a  mechanical  mixture  of  sulphur,  char- 
coal, and  saltpeter  generally  in  the  proportions  of  10  parts  sulphur, 
15  parts  charcoal,  and  75  parts  saltpeter  (sodium  nitrate).  It 
weighs  about  62J  pounds  per  cubic  foot  and  produces  about  280 
times  its  own  volume  in  gas  at  a  pressure  of  4.68  tons  per  square 
inch  at  a  temperature  of  32  degrees  F.,  which  amounts  to  a  pres- 
sure of  approximately  38  tons  per  square  inch  at  the  temperature 
of  explosion  of  4,000  degrees  F. 

Blasting  Powder. — This  is  a  mixture  of  19  parts  sulphur,  15 
parts  charcoal,  and  66  parts  saltpeter.  These  powders  are  made 
in  different  size  angular  polished  grains,  from  the  size  of  a  pin 
head  to  sizes  just  passing  a  f  to  \  inch  hole.  The  larger  the  grains 
the  slower  the  action  of  the  powder. 

Nitro-Substitution  Compounds. — These  compounds  are  formed 
by  the  action  of  nitric  acid  on  hydro-carbons.  Triton,  T.N.T., 
or  trinitrotoluene,  made  famous  during  the  war,  is  an  example  of 
these  compounds.  It  is  made  by  the  successive  nitration  of 
toluene,  a  coal  tar  derivative.  It  melts  at  80  degrees  C.,  is  very 
stable,  and  is  of  great  explosive  strength.  It  is  manufactured  in 
a  convenient  form,  being  compressed  into  blocks  about  2  inches 
square  by  about  4  inches  long  with  a  specific  gravity  of  about  1.5. 
The  blocks  are  usually  copper  plated  to  protect  the  T.N.T.  from 
moisture.  The  more  dense  it  is  the  less  its  sensitiveness.  It  is 
also  put  up  in  crystalline  form  in  cartridges  like  dynamite,  in 
which  condition  it  is  practically  equal  to  40  per  cent  dynamite. 
It  can  be  cut  with  a  knife,  pounded  with  a  hammer,  and  will  burn 
freely  and  slowly  in  small  quantities  in  the  open  air  without 
exploding.  It  is  suitable  for  all  but  the  hardest  rocks.  It  creates 
poisonous  gases  on  detonation  which  are  quickly  dissipated  in  the 
open  air  but  which  render  it  unsuitable  for  use  in  tunnel  work. 

Nitro-glycerine. — This  is  formed  by  the  action  of  nitric  and 
sulphuric  acids  on  animal  compounds  such  as  gelatine  or  glycerine. 
Nitro-glycerine  is  a  yellowish,  oily,  highly  unstable  explosive 
liquid  with  a  specific  gravity  of  about  1.6.  It  will  burn  quietly 
when  ignited  in  the  open  air.  It  will  freeze  at  41  degrees  F.,  and 
will  explode  at  388  degrees  F.,  or  on  concussion  at  a  lower  tempera- 
ture. It  develops  about  1,500  times  its  volume  in  gas,  which  due 
to  the  heat  of  combustion  is  increased  to  about  10,000  times  its 
volume.  It  is  a  very  dangerous  explosive  to  handle,  and  is  unsuit- 
able for  use  in  the  liquid  form. 


296  CONSTRUCTION 

Blasting  Gelatine. — This  is  made  by  soaking  guncotton  in 
nitre-glycerine.  Gelatine  dynamite,  is  a  combination  of  blasting 
gelatine  and  an  absorbent.  Forcite  is  a  gelatine  dynamite  in 
which  the  blasting  gelatine,  forming  50  per  cent  of  the  compound, 
contains  90  per  cent  nitro-glycerine  and  2  per  cent  guncotton; 
and  the  absorbent,  forming  the  other  50  per  cent  of  the  compound, 
contains  76  per  cent  of  sodium  nitrate,  3  per  cent  sulphur,  20  per 
cent  of  wood  tar,  and  1  per  cent  of  wood  pulp. 

Blasting  gelatine  is  packed  in  a  jelly-like  mass  in  metal  lined 
wooden  boxes.  It  is  less  sensitive  than  straight  dynamite  and  is 
one  of  the  most  powerful  explosives  known.  It  can  be  made  up 
to  equal  100  per  cent  dynamite.  It  is  suitable  for  use  in  the  hard- 
est rocks  and  for  subaqueous  work  as  it  is  not  affected  by  moisture. 
It  is  suitable  for  use  in  tunnels  as  the  amount  of  carbon  monoxide, 
peroxide  of  nitrogen,  hydrogen  sulphide  and  other  dangerous 
gases  is  comparatively  low  when  fully  detonated.  Gelatine 
dynamite  1  is  sold  as  30  per  cent  to  70  per  cent  dynamite,  the 
actual  percentage  of  nitro-glycerine  being  less  than  the  nominal 
quantity  given. 

Dynamite. — The  dynamites  are  made  by  soaking  nitro-glycerine 
in  some  absorbent.  If  the  absorbent  is  some  neutral  substance 
such  as  infusorial  earth  the  combination  is  known  as  a  true  dyna- 
mite. The  false  or  active  dynamites  are  those  in  which  the  absorb- 
ent is  also  an  explosive  compound.  The  false  dynamites  form  the 
best  known  contractors'  explosives.  Among  the  materials  mixed 
with  the  nitro-glycerine  are:  magnesium  carbonate,  sulphur, 
wood  meal,  wood  pulp,  wood  fiber,  wood  tar,  nut  galls,  kieselguhr, 
sawdust,  resin,  pitch,  sugar,  charcoal,  and  guncotton.  The 
strength  of  dynamites  is  noted  by  the  per  cent  of  nitro-glycerine 
and  nitro  substitutes  contained.  Dualin  and  Hercules  powder 
both  contain  40  per  cent  nitro-glycerine.  Dualin  contains  30 
per  cent  sawdust  and  30  per  cent  potassium  nitrate,  but  the 
Hercules  powder,  which  is  stronger,  contains  16  per  cent  sugar, 
3  per  cent  potassium  chlorate,  31  per  cent  potassium  nitrate,  and 
10  per  cent  magnesium  carbonate. 

Dynamite  is  the  most  common  explosive  used  on  construction 

work.     It  is  supplied  in  cylindrical  sticks  wrapped  in  paper,  the 

diameter  of  the  sticks  varying  between  |  and  2  inches.     They  are 

about  8  inches  long.     Forty  per  cent  dynamite  is  the  common 

1  See  paper  by  C.  T.  Hall  before  Am.  Inst.  Chemical  Engineers. 


PERMISSIBLE  EXPLOSIVES  297 

strength  found  on  the  market.  It  is  suitable  for  ordinary  work 
in  all  but  very  hard  rocks  or  very  soft  material.  Direct  contact 
with  water  separates  the  nitro-glycerine  from  the  base  and  is 
dangerous  when  the  explosive  is  used  in  wet  places  unless  it  is 
fired  immediately  after  the  hole  is  loaded.  It  freezes  at  about  42 
degrees  F.,  or  at  even  higher  temperatures  and  in  the  frozen  state 
it  is  highly  dangerous,  requiring  powerful  detonators  for  firing, 
but  exploding  spontaneously  from  a  slight  jar,  or  the  breaking  of 
the  stick.  Special  low-freezing  dynamites  are  made  that  will  not 
freeze  above  35  degrees  F. 

Ammonia  Compounds. — Ammonia  dynamite  is  a  combination 
of  nitro-glycerine,  ammonium  nitrate  and  such  other  ingredients 
as  sodium  nitrate,  calcium  carbonate  and  combustible  material. 
This  form  of  explosive  is  advantageous  for  underground  work 
because,  like  gelatine  dynamite,  its  explosion  does  not  create  large 
quantities  of  poisonous  gases.  It  has  a  low  freezing  point  and  is 
relatively  low  in  cost.  It  is  seriously  affected  by  moisture,  however, 
and  can  not  be  used  in  wet  places.  Ammonium  nitrate  explosives 
which  do  not  contain  nitro-glycerine  include  70  per  cent  to  95  per 
cent  ammonium  nitrate  and  some  combustible  material.  Ammo- 
nal is  a  special  type  of  this  class  formed  by  a  mixture  of  ammo- 
nium nitrate,  aluminum,  and  triton.  All  of  these  explosives  are 
deliquescent,  insensitive  to  shock,  and  are  cheaper  than  the  dyna- 
mites. 

173.  Permissible    Explosives. — As   specified    by   the    United 
States  Bureau  of  Mines  explosives  whose  rapidity,  detonation, 
and  temperature  of  explosion  will  not  ignite  explosive  mixtures 
of  pit  gases  and  air  are  known  as  permissible  explosives.     They 
include  nitrate  explosives,  ammonia  dynamite,  and  others. 

Gunpowder,  triton,  picric  acid,  blasting  gelatine,  dynamite, 
guncotton,  etc.,  are  not  classed  as  permissible  explosives. 

174.  Strength. — The  relative  weights  for  equal  strength  of 
various  explosives  are  given  in  Table  62. 

175.  Fuses  and  Detonators. — The  explosion  of  gunpowder  and 
other  deflagrating  explosives  is  caused  by  the 'direct  application 
of  a  flame  led  to  the  charge  by  a  powder  fuse,  or  they  may  be 
fired  by  a  blasting  cap  which  is  itself  exploded  by  the  heat  from 
a  fuse  or  an  electric  spark.     The  powder  fuse  is  a  cord  made  up  of 
a  train  of  powder  securely  wrapped  in  a  number  of  thicknesses  of 
woven  cotton  or  linen  threads  and  usually  made  water-proof. 


CONSTRUCTION 


TABLE  62 

RELATIVE  WEIGHTS  OF  EXPLOSIVES  WITH  THE  SAME  STRENGTH  AS  A 
UNIT  WEIGHT  OF  40  PER  CENT  DYNAMITE 


Explosive 

Relative 
Weight 

Explosive 

Relative 
Weight 

Picric  acid  

0.86 

Triton 

0.86 

Gun  powder  (well  tamped)  . 

3.10 

Blasting  gelatine  

0.43 

Straight  dynamite,  15%.  ... 

1.45 

Gelatine  dynamite,  30%.  .  . 

1.28 

Straight  dynamite,  20     .... 

1.33 

Gelatine  dynamite,  35 

1.21 

Straight  dynamite,  25     .... 

1.28 

Gelatine  dynamite,  40     ... 

1.14 

Straight  dynamite,  30     .... 

1.18 

Gelatine  dynamite,  50 

1.04 

Straight  dynamite,  35     .... 

1.07 

Gelatine  dynamite,  55     ... 

0.97 

Straight  dynamite,  40     .... 

1.00 

Gelatine  dynamite,  60     ... 

0.90 

Straight  dynamite,  45     .... 

0.93 

Gelatine  dynamite,  70 

0.83 

Straight  dynamite,  50     .... 

0.86 

Straight  dynamite,  55     .... 

0.83 

Ammonia    dynamites    are 

Straight  dynamite,  60     .... 

0.78 

the  same  as  gelatine 

dynamites 

Low-freezing  dynamites  are 

Chlorates  (sprengle) 

the    same    as    straight 

Rack-a-rock  

1.33 

dynamites 

Guncotton  

0.72 

Smokeless      powder,      well 

tamped  

0.74 

Ordinary  fuse  burns  at  about  2  feet  per  minute  but  there  may  be 
wide  variations  from  this  rate  due  to  the  quality  of  the  fuse, 
moisture,  temperature,  or  pressure.  Moisture  tends  to  retard  the 
rate,  pressure  to  increase  it.  Instantaneous  fuse  will  burn  at 
about  120  feet  per  second.  It  is  distinguished  from  the  ordinary 
safety  fuse  both  by  eye  and  touch  due  to  the  rough  red  braid  with 
which  it  is  covered.  It  is  used  in  firing  a  number  of  charges 
simultaneously.  Powder  fuses  are  lighted  by  the  application  of  a 
flame  or  smoldering  torch  to  the  freshly  cut  or  opened  end  expos- 
ing the  powder  grains.  Cordeau  Bickford  is  lead  tubing  filled  with 
triton,  in  which  the  flame  travels  at  about  17,000  feet  per  second. 
This  is  also  used  for  igniting  charges  simultaneously. 

The  detonation  of  an  explosive  is  caused  by  the  shock  or  heat 
of  the  explosion  of  a  more  sensitive  substance  which  has  been 
exploded  by  a  powder  fuse  or  electric  spark.  The  common 
method  of  detonating  explosive  charges  is  by  the  firing  of  a  blast- 


FUSES  AND  DETONATORS 


ing  cap.  These  caps  are  copper  cylinders,  closed  at  one  end, 
about  1J  inches  long  and  J  to  |  of  an  inch  in  diameter,  or  larger. 
They  contain  a  mixture  of  about  85  per  cent  fulminate  of  mercury 
and  15  per  cent  potassium  chlorate  held  in  place  by  a  wad  of 
shellac,  collodion,  or  paper.  The  strength  of  detonators  is  based 
on  the  weight  of  fulminate  of  mercury  and  is  designated  as  shown 
in  Table  63. 


TABLE  63 

STRENGTH  OF  BLASTING  CAPS 


Grains 

Grains 

Blasting  Cap, 

Fulminate 

Electric  Cap, 

Fulminate 

Commercial  Grade 

of 

Commercial  Grade 

of 

Mercury 

Mercury 

3X  or  Triple  

8.3 

Single  strength 

12  3 

4X  or  Quadruple  

10.0 

Double  strength  

15.4 

5X  or  Quintiple 

12  3 

Triple  strength 

23  1 

6X  or  Sextuple  

15.4 

Quadruple  strength 

30  9 

7X  or  Number  20  

23.1 

8X  or  Number  30  

30.9 

The  force  of  the  explosion  is  markedly  affected  by  the  strength 
of  the  caps,  the  effect  being  greater  for  low-grade  powders.  For 
40  per  cent  dynamite  the  explosion  caused  by  a  5X  cap  is  15  per 
cent  stronger  than  that  caused  by  a  3X  cap.  For  60  per  cent 
dynamite  the  difference  is  only  6  per  cent.  The  deterioration  of 
the  caps  will  reduce  the  strength  of  an  explosion  noticeably. 
With  straight  dynamite,  3X  caps  are  generally  used,  but  with 
gelatine  dynamite  6X  or  heavier  caps  must  be  used.  Caps  may 
be  tested  by  exploding  them  in  a  confined  space  and  noting  the 
report  and  the  effect  on  the  shell.  A  full  strength  cap  will  tear 
the  shell  into  minute  pieces,  while  a  deteriorated  cap  will  merely 
tear  it  into  three  or  four  large  pieces.  An  ordinary  blasting  cap  is 
shown  in  Fig.  120  together  with  other  equipment  for  blasting. 

Firing  by  electricity  is  generally  safer  and  more  satisfactory 
than  by  the  use  of  ordinary  caps  and  powder  fuses.  The  explosion 
is  more  certain  and  its  exact  time  is  under  the  control  of  the  opera- 
tor. Fig.  121  shows  a  section  through  an  electric  blasting  cap  or 


300 


CONSTRUCTION 


detonator,  commonly  called  an  electric  fuse.  Delayed-action 
electric  detonators  are  made  by  inserting  a  slow-burning  sub- 
stance between  the  platinum  bridge  and  the  detonating  substance. 
The  time  of  delay  is  controlled  by  the  depth  of  the  slow -burning 
substance.  Delayed-action  detonators  are  useful  in  tunnel  work 
where  it  is  desired  to  explode  the  charge  in  three  or  four  stages 
in  order  that  the  debris  from  one  charge  may  be  out  of  the 
way  of  the  following,  and  that  the  forces  of  the  explosions  may 
not  serve  to  nullify  each  other. 

176.  Care  in  Handling. — Some  of  the  don'ts  in  the  handling 


MAGNETO 


DYNAMI1E  .CARTRIDGES 


POWDER 


ELECTRI 
CAPS 

CLECTRK 
FUSE 

BLASTING 
CAPS 


FIISE 

REEL 


FUSE 


ELECTRIC  WIRE 


CAP  CRIMPERS      SAFETY'    RESISTANCE    x  GALVANOMETER 
FUSE  COIL 


FIG.  120.— Blasting  Supplies. 

Courtesy,  Aetna  Powder  Co. 

of  explosives  recommended  by  the  U.  S.  Army  Engineer  Field 
Manual  are :  in  the  use  of  nitro-glycerine  explosives  of  all  kinds — 

(a)  Don't  store  detonators  with  explosives.     Detonators 
should  be  kept  by  themselves. 

(6)  Don't  open  packages  of  explosives  in  a  store  house. 

(c)  Don't  open  packages  of  explosives  with  a  nail  puller, 
pick  or  chisel.     Packages  should  be  opened  with  a  hard 
wood  wedge  and  mallet,  outside  of  the  magazine  and  at 
some  distance  from  it. 

(d)  Don't  store  explosives  in  a  hot  or  damp  place.     All 
explosives  spoil  rapidly  if  so  stored. 

(e)  Don't  store  explosives  containing  nitro-glycerine  so 
that  the  cartridges  stand  on  end.     The  nitro-glycerine  is 
more  likely  to  leak  from  the  cartridges  when  they  stand  on 
end  than  it  is  when  they  lie  on  their  sides. 


CARE  IN  HANDLING 


301 


(/)  Don't  use  explosives  that  are  frozen  or  partly 
frozen.  The  charge  may  not  explode  completely  and  seri- 
ous accidents  may  result.  If  the  explosion  is  not  complete 
the  full  strength  of  the  charge  is  not  exerted  and  larger 
quantities  of  harmful  gases  are  given  off. 

(g)  Don't  thaw  frozen  explo- 
sives in  front  of  an  open  fire,  nor  in 
a  stove,  nor  over  a  lamp,  nor  near 
a  boiler,  nor  near  steam  pipes,  nor 
by  placing  cartridges  in  hot  water. 
Use  a  commercial  or  improvised 
thawer. 

(h)  Don't  put  hot  water  or 
steam  pipes  in  a  magazine  for 
thawing  purposes. 

(i)  Don't  carry  detonators  and 
explosives  in  the  same  package. 
Detonators  are  extremely  sensitive 
to  heat,  friction,  or  blows  of  any 
kind. 

(.7)  Don't  handle  detonators  or 
explosives  near  an  open  flame. 

(k)  Don't  expose  detonators  or 
explosives  to  direct  sunlight  for 
any  length  of  time.  Such  exposure 
may  increase  the  danger  in  their 
use. 

(I)  Don't  open  a  package  of  explosives  until  ready  to 
use  the  explosive,  then  use  it  promptly. 

(m)  Don't  handle  explosives  carelessly.  They  are  all 
sensitive  to  blows,  friction,  and  fire. 

(ri)  Don't  crimp  a  detonator  (blasting  cap)  around  a 
fuse  with  the  teeth.  Use  a  cap  crimper,  which  is  supplied 
for  this  purpose. 

(o)  Don't  economize  by  using  a  short  length  of  fuse. 

(p)  Don't  return  to  a  charge  for  at  least  one-half  hour 
after  a  miss  fire.  Hang  fires  are  likely  to  happen. 

(q)  Don't  attempt  to  draw  nor  to  dig  out  the  charge  in 
case  of  a  miss  fire. 

Some  of  the  positive  rules  in  connection  with  the  handling  of 
explosives  are :  build  the  magazine  on  an  earth  foundation  remote 
from  any  other  structures,  protect  it  with  earth  embankments 
that  will  direct  the  force  of  the  explosion  upwards,  and  build  it 
of  materials  that  will  supply  as  few  missiles  as  possible.  Hollow 
tile  brick,  double- walled  galvanized  iron  filled  with  sand,  and 
similar  constructions  are  satisfactory.  The  magazine  may  be 


•-  Electric  Leads 


•Copper  Shell 


[.•Sulphur  Filling 


-Sulphur  Plug 

--Platinum  Bridge 

-Guncotton  or  Loose 
Mercury  Fulminate 


-Mercury  Fulminate 
Packed 


FIG.  121.— Electric  Fuse. 

Full  size. 


302 


CONSTRUCTION 


heated  by  steam  or  hot-water  pipes  so  located  that  explosives  can- 
not come  in  contact  with  them,  or  by  a  cluster  of  incandescent 
bulbs,  but  if  the  explosives  become  frozen  they  must  not  be  thawed 
out  by  turning  on  the  steam  or  hot  water.  If  powder  or  nitro- 
glycerine is  dropped  on  the  floor  the  magazine  should  be  emptied, 
washed  out  with  a  hose  and  spots  of  nitro-glycerine  scrubbed  with 
a  brush  and  a  mixture  of  J  gallon  of  wood  alcohol,  J  gallon  of 
water  and  2  pounds  of  sodium  sulphide.  Frozen  explosives  may 
be  thawed  by  spreading  out  on  special  shelves  in  a  warm  thaw 
house — not  in  the  magazine  proper,  by  burying  in  a  manure  pile 
so  that  the  explosive  may  not  become  moistened,  or  more  com- 
monly by  heating  slowly  in  a  water  bath.  This  is  a  dry  kettle  in 
which  the  explosives  are  placed  and  covered.  The  kettle  is  then 
put  in  another  containing  water  which  is  heated  gently  to  about 
120  degrees  F.  It  should  not  be  boiled. 

In  case  of  a  miss  fire,  instead  of  digging  out  the  old  charge  put 
a  new  charge  on  top  of  the  old  and  fire  the  two 
simultaneously. 

177.  Priming,  Loading,  and  Firing. — Priming  is 
the  act  of  placing  the  cap  or  detonator  in  the  cart- 
ridge of  explosive.  The  primer  is  either  the  cap  or 
the  cap  and  cartridge  which  are  to  be  detonated  by 
the  fuse.  If  a  cap  and  safety  fuse  are  to  be  used  the 
paper  at  the  upper  end  of  the  cartridge  is  opened,  a 
hole  is  poked  in  the  explosive  with  the  finger  or  a 
piece  of  wood,  the  cap  and  the  attached  fuse  are 
pushed  into  the  hole  and  gently  embedded  in  the 
explosive  so  that  the  end  of  the  cap  is  exposed 
sufficiently  to  prevent  the  fuse  from  igniting  the 
dynamite  directly.  The  paper  is  then  folded  up 
and  tied  firmly  around  the  fuse  with  a  piece  of 
string.  The  result  is  shown  in  Fig.  122. 

In  placing  the  fuse  in   the   cap  the  end  of   the 
fuse  is  cut   off   square,   and  inserted  in  the  open 
ar  en<^  °^  ^e  caP'  care  being  taken  not  to  spill  the 

Safety  Fuse,  loose  grains  of  powder  or  to  grind  the  fuse  down 
and  Cap.       on  top    of   the    cap.      When   the   fuse   is    shoved 
firmly  into  place  the  upper  portion  of  the  copper 
cap  is  pressed  or  crimped  with  the  cap  crimpers  shown  in  Fig.  120. 
The  number  of  primers  to  be  used  is  dependent  on  the  size 


FIG 


PRIMING,   LOADING,   AND  FIRING  303 

and  location  of  the  charge,  but  in  practically  all  sewer  work  only 
one  primer  is  used  to  each  hole.  In  bulky  charges  the  primer 
should  be  placed  near  the  center  of  the  charge  and  the  fuse  so 
protected  that  it  will  not  ignite  the  charge  prematurely.  In  drill 
holes  the  primer  is  put  in  last  with  the  cap  end  down. 

In  loading  a  hole,  it  is  first  pumped  and  cleaned  out.  This 
can  be  done  satisfactorily  with  the  end  of  a  stick  frayed  out  into  a 
broom.  Cartridges  which  very  nearly  fill  the  hole  are  dropped  in 
one  at  a  time  and  are  pressed  firmly  together,  with  a  light  wooden 
tamping  bar.  They  should  not  be  pounded.  After  the  primer 
is  placed,  a  wad  of  clay  or  similar  material  is  pressed  gently  into 
the  hole  against  it  and  the  hole  is  then  filled  with  well-tamped  clay. 
In  tunnel  work  tamping  is  not  so  essential  as  an  overcharge  of 
powder  is  usually  used  and  the  time  of  tamping,  which  is  worth 
more  than  two  or  three  sticks  of  dynamite,  is  saved.  In  handling 
bulk  explosives,  such  as  gunpowder,  they  are  poured  into  the  hole, 
the  fuse  is  set  in  the  upper  portion  and  the  remainder  of  the  hole 
is  tamped  with  clay  as  for  dynamite  cartridges. 

If  a  large  number  of  charges  are  to  be  fired  simultaneously 
with  a  safety  fuse,  the  length  of  the  fuse  to  each  charge  should  be 
made  equal  or  a  safety 
fuse  used  to  a  common 
center    and    approxi- 
mately equal   lengths 
of  instantaneous  fuse 
or  Cordeau   Bickford 
used    from    there    to     FIG.  123.— Methods  for  Cutting  Safety  Fuse  for 
the  charge.     In  splic-  Splicing, 

ing  the  fuses  for  such 

connections  they  are  cut  diagonally  as  shown  in  Fig.  123  and  bound 
together  firmly  with  tape.  Electric  connections  are  particularly 
advantageous  under  such  conditions  as  they  avoid  the  dangers 
incidental  to  spliced  fuses  and  are  less  expensive.  In  tunnel 
work  simultaneous  electric  detonation  is  not  desirable  as  the  holes 
should  be  fired  progressively:  1st,  the  cuts;  2nd,  the  relievers; 
3rd,  the  backs;  4th,  the  sides;  and  5th,  the  lifters.  Different 
lengths  of  safety  fuse,  or  delayed  action  electric  fuses  can  be  used 
for  these  delay  shots. 

In  igniting  a  safety  fuse  an  open  flame  such  as  that  furnished 
by  a  match  or  candle  is  the  most  satisfactory.  For  electric  fuses 


304  CONSTRUCTION 

the  current  is  generated  by  a  magneto  shown  in  Fig.  120. 
Pressing  vigorously  down  on  the  handle  closes  the  circuit  and 
generates  an  electric  current  which  heats  the  platinum  bridges 
and  explodes  the  charges.  For  the  small  number  of  charges  used 
in  ordinary  construction  they  are  connected  in  series  so  that  if 
there  is  a  broken  connection  anywhere  no  charge  will  be  exploded. 
If  many  charges  are  to  be  fired  and  a  line  circuit  is  to  be  used,  the 
final  connection  should  not  be  made  until  just  before  the  charge 
is  to  be  fired  in  order  to  obviate  the  danger  of  stray  currents  firing 
the  charge  prematurely.  Care  should  be  taken  to  see  that  all 
connections  are  good  and  that  there  are  no  broken  wires  on  the  line. 

178.  Quantity  of  Explosive. — The  quantity  of  explosive  to 
be  used  can  be  determined  satisfactorily  only  by  experience  on 
the  job  in  question,  as  the  factors  affecting  the  necessary  quantity 
are  so  diverse.     The  figures  in  Table  64  indicate  the  relative 
amounts  needed  under  different  conditions. 

PIPE  SEWERS 

179.  The  Trench  Bottom. — It  is  customary  to  dig  the  bottom 
of  the  trench  to  conform  to  the  shape  of  the  lower  45  degrees 
to  90  degrees  of  the  sewer  if  the  character  of  the  material  will 
allow  such  construction.     In  soft  material  which  will  not  hold 
its  shape  the  sewer  may  be  encased  in  concrete  or  a  concrete 
cradle  may  be  prepared  for  the  pipe.     In  rock  the  trench  is 
excavated  to  about  6  inches  below  grade  and  refilled  with  well- 
tamped  earth  so  as  to  form  a  cradle  giving  bearing  to  60  to  90 
degrees  of  the  pipe  circumference.     For  large  sewers  to  be  con- 
structed in  the  trench  special  foundations  are  sometimes  built. 

180.  Laying  Pipe. — Before  the  pipe  is  lowered  into  the  trench 
the  sections  which  are  to  be  adjacent  should  be  fitted  together 
on  the  surface  and  the  relative  positions  marked  by  chalk  so  that 
the  same  position  can  be  obtained  in  the  trench. 

Small  pipes  are  lowered  into  the  trench  and  swung  into  posi- 
tion on  a  hook  as  shown  in  Fig.  124.  Pipes  up  to  15  or  18  inches 
in  diameter  can  be  handled  by  the  pipe  layer  and  helper  in  the 
trench  without  assistance.  Heavier  pipes  may  be  lowered  into 
the  trench  by  passing  ropes  around  each  end  of  the  pipe.  One 
end  of  the  rope  is  fastened  at  the  surface  and  the  ropes  are  paid 
out  by  the  men  at  the  surface  as  the  pipe  is  lowered.  If  the  pipes 


QUANTITY  OF  EXPLOSIVE 


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306  CONSTRUCTION 

have  been  fitted  together  and  marked  at  the  surface  it  is  undesir- 
able to  use  this  method  of  lowering  as  the  position  in  which  the 
pipes  arrive  in  the  bottom  of  the  trench  can  not  be  easily  pre- 
dicted. A  cradle  may  be  used  for  shoving  the  pipe  into  position 
as  is  shown  in  Fig.  125. 

Pipes  above  24  to  27  inches  in  diameter  are  too  large  to  be 
handled  from  the  side  of  the  trench.  A  hook  as  shown  in  Fig. 
124  is  placed  in  the  pipe  so  that  it  will  be  in  the  proper  position 
when  lowered.  It  is  raised  by  a  rope  passing  through  a  block 
at  the  peak  of  a  stiff-legged  derrick  which  spans  the  trench,  or 
by  a  crane.  If  a  derrick  is  used  the  rope  passes  to  a  windlass 
on  the  opposite  side  of  the  trench  from  the  pipe.  Mechanical 


FIG.  124. — Hook  for  Lowering  and  FIG.  125. — Cradle  for  Placing 

Placing  Sewer  Pipe.  Sewer  Pipe. 

power  may  be  used  for  raising  pipes  too  heavy  to  be  raised  by  hand. 
The  pipe  is  then  lowered  and  swung  into  position  while  sup- 
ported from  the  derrick.  Excessive  swinging  is  prevented  by 
holding  back  on  the  guide  rope  as  the  pipe  is  raised  and  lowered. 
Pipes  are  usually  laid  with  the  bell  end  up  grade  as  it  is  easier 
to  fit  the  succeeding  pipe  into  the  bell  so  laid  and  to  make  the 
joint,  particularly  on  steep  grades.  The  Baltimore  specifica- 
tions state: 

The  ends  of  the  pipe  shall  abut  against  each  other  in 
such  a  manner  that  there  shall  be  no  shoulder  or  unevenness 
of  any  kind  along  the  inside  of  the  bottom  half  of  the 
sewer  or  drain.  Special  care  should  be  taken  that  the 

pipe    are   well  bedded  on  a  solid  foundation The 

trenches  where  pipe  laying  is  in  progress  shall  be  kept  dry, 
and  no  pipe  shall  be  laid  in  water  or  upon  a  wet  bed  unless 
especially  allowed  in  writing  by  the  Engineer.  As  the 
pipe  are  laid  throughout  the  work  they  must  be  thoroughly 
cleaned  and  protected  from  dirt  and  water,  no  water  being 
allowed  to  flow  in  them  in  any  case  during  the  construction 
except  such  as  may  be  permitted  in  writing  by  the  Engineer. 
No  length  of  pipe  shall  be  laid  until  the  preceding  length 
has  been  thoroughly  embedded  and  secured  in  place,  so  as 
to  prevent  any  movement  or  disturbance  of  the  finished 
joint. 


/  JOINTS  307 

The  mouth  of  the  pipe  shall  be  provided  with  a  board 
or  stopper,  carefully  fitted  to  the  pipe,  to  prevent  all  earth 
and  any  other  substances  from  washing  in. 

181.  Joints. — Pipes  may  be  laid  with  open  joints,  mortar 
joints,  cement  joints,  or  poured  joints.  Open  joints. are  used  for 
storm  sewers  in  dry  ground  close  to  the  surface.  Mortar  and 
cement  joints  are  commonly  used  on  all  sewers  except  in  special 
cases.  Cement  joints  are  more  carefully  made  than  mortar 
joints  and  result  in  a  greater  percentage  of  water-tight  joints. 
Poured  joints  are  used  in  wet  trenches  where  it  is  necessary  to 
exclude  ground  water  from  the  sewer. 

A  specification  used  in  some  cities  for  open  joints  is: 

Pipes  laid  with  open  joints  are  to  be  laid  with  their 
inverts  in  the  same  straight  line  and  shall  be  firmly  bedded 
throughout  their  length  on  the  bottom  of  the  trench.  No 
cement  or  mortar  is  to  be  used  in  the  joints.  Not  more 
than  |  inch  shall  be  left  between  the  spigot  end  of  the  pipe 
and  the  shoulder  of  the  hub  of  the  pipe  into  which  it  fits. 
The  joints  shall  be  surrounded  with  cheese  cloth,  burlap, 
broken  pipe,  gravel  or  broken  stone. 

The  purpose  of  the  cheese  cloth,  etc.,  is  to  prevent  fine  earth 
from  sifting  into  the  pipe  until  the  cheese  cloth  or  other  material 
has  rotted  away,  by  which  time  the  earth  has  become  arched  over 
the  opening. 

Mortar  joints  are  specified  by  Metcalf  and  Eddy  as  follows: 

Before  a  pipe  is  laid  the  lower  part  of  the  bell  of  the 
preceding  pipe  shall  be  plastered  on  the  inside  with  stiff 
mortar  of  equal  parts  of  Portland  cement  and  sand,  of 
sufficient  thickness  to  bring  the  inner  bottoms  of  the 
abutting  pipe  flush  and  even.  After  the  pipe  is  laid  the 
remainder  of  the  bell  shall  be  thoroughly  filled  with  similar 
mortar  and  the  joint  wiped  inside  and  finished  to  a  smooth 
bevel  outside. 

In  some  work  a  wood  block  or  a  stone  is  embedded  in  the  mor- 
tar at  the  bottom  of  the  joint  to  bring  the  spigot  in  place  concen- 
tric with  the  next  pipe. 

Cement  joints  are  specified  in  the  Baltimore  specifications  as 
follows: 

Cement  joints  shall  be  made  with  a  narrow  gasket  of 
hemp  or  jute  and  cement  mortar,  and  special  care  shall  be 
taken  to  secure  tight  joints.  The  gasket  jshall  be  soaked 


308  CONSTRUCTION 

in  Portland  cement  grout  and  then  carefully  inserted 
between  the  bell  and  the  spigot,  and  well  calked  with 
suitable  hardwood  or  iron  calking  tools.  It  shall  be  in 
one  continuous  piece  for  each  joint,  and  of  such  thickness 
as  to  bring  the  inverts  of  the  two  pipes  smooth  and  even. 
The  remainder  of  the  joint  shall  be  filled  with  cement  mortar 
all  around,  on  the  bottom,  top  and  sides,  applied  by  hand 
with  rubber  mittens,  well  pressed  into  the  annular  space 
and  beveled  off  from  the  outer  edge  of  the  bell  to  a  dis- 
tance of  two  inches  therefrom,  or  to  an  angle  of  45  degrees. 
The  inside  of  each  joint  shall  be  thoroughly  cleansed  of  all 
surplus  mortar  that  may  squeeze  out  in  making  the  joint; 
and  to  accomplish  this  some  suitable  scraper  or  follower, 
or  form  shall  be  provided  and  always  used  immediately 
after  each  joint  is  finished. 

Cement  joints  so  made,  form  the  most  satisfactory  joint 
for  ordinary  conditions  and  are  the  most  frequently  used.  They 
are  not  always  water-tight  and  can  be  penetrated  by  roots.  Some 
roots  are  able  to  penetrate  holes  of  almost  microscopic  size  and 
to  form  growths  in  the  sewer  or  to  split  the  joints. 

Poured  joints  are  made  by  pouring  some  jointing  compound, 
while  in  a  fluid  state,  into  the  joint  in  which  it  hardens,  thus  seal- 
ing the  joint.  Water-tightness  in  sewer  lines  to  exclude  ground 
water  has  also  been  attempted  by  using  the  ordinary  cement  joint 
and  surrounding  the  pipe  with  a  layer  of  cement  or  concrete. 
This  has  not  always  been  successful  as  it  is  difficult  to  obtain  the 
proper  class  of  workmanship  in  wet  sewer  trenches. 

The  requisite  qualities  of  a  poured  jointing  material  are: 

(1)  It  should  make  a  joint  proof  against  the  entrance 
of  water  and  roots. 

(2)  It  should  be  inexpensive. 

(3)  It  should  have  a  long  life. 

(4)  It  should  not  deteriorate  in  sewage  which  may  be 
either  acid  or  alkaline. 

(5)  It  should  adhere  to  the  surface  of  the  pipe. 

(6)  It  should  run  at  a  temperature  below  about  400°  F., 
as  too  high  temperatures  will  crack  the  pipe. 

(7)  It  should  neither  melt  nor  soften  at  temperatures 
below  250°  F.  in  order  to  maintain  the  joint  if  hot  liquids 
are  poured  into  the  sewer. 

(8)  It  should  be  elastic  enough  to  permit  slight  move- 
ments of  the  pipes. 

(9)  It  should  not  require  great  skill  in  using  as  it  must 
be  handled  ordinarily  by  unskilled  workers. 


JOINTS  309 

/ 

The  materials  used  for  poured  joints  are:  cement  grout; 
sulphur  and  sand;  and  asphalt  or  some  bituminous  compound 
made  of  vulcanized  linseed  oil,  clay,  and  other  substances  the 
resulting  mixture  having  the  appearance  of  vulcanized  rubber 
or  coal  tar.  The  bituminous  materials  most  nearly  approach 
the  ideal  conditions. 

Cement  grout  is  made  up  of  pure  cement  and  water  mixed 
into  a  soupy  consistency.  Its  main  advantages  are  its  cheapness 
and  ease  in  .handling  in  wet  trenches  or  difficult  situations.  The 
result  is  no  better  than  a  well  made  cement  joint.  There  is  no 
elasticity  to  the  joint  and  a  movement  of  the  pipe  will 
break  it. 

Sulphur  and  sand  are  inexpensive,  comparatively  easy  to 
handle,  and  make  an  absolutely  water-tight  and  rigid  joint  which 
is  stronger  than  the  pipe  itself.  It  frequently  results  in  the  crack- 
ing of  the  pipe  and  is  objected  to  by  some  engineers  on  that 
account.  In  making  the  mixture,  powdered  sulphur  and  very 
fine  sand  are  mixed  in  equal  proportions.  It  is  essential  that  the 
sand  be  fine  so  that  it  will  mix  well  with  the  sulphur  and  not 
precipitate  out  when  the  sulphur  is  melted.  Ninety  per  cent 
of  the  sand  should  pass  a  No.  100  sieve  and  50  per  cent  should  pass 
a  No.  200  sieve.  The  mixture  melts  at  about  260°  F.  and  dees 
not  soften  at  lower  temperatures.  For  making  a  joint  in  an  8 
inch  pipe  about  1J  pounds  of  sulphur,  1^  pounds  of  sand,  \ 
pound  of  jute,  and  0.4  pound  of  pitch  are  used.  The  pitch  is 
used  to  paint  the  surface  of  the  joint  while  still  hot  in  order  to 
close  up  any  possible  cracks. 

Among  the  better  known  of  the  bituminous  joint  compounds 
are:  "  O.K."  Compound  made  by  the  Atlas  Company,  Mertz- 
town,  Pa.,  Jointite  and  Filtite,  manufactured  by  the  Pacific  Flush 
Tank  Co.,  Chicago  and  New  York,  and  some  of  the  products  of 
the  Warren  Brothers  Co.,  Boston.  These  compounds  fill  nearly 
all  of  the  ideal  conditions  except  as  to  cost  and  ease  in  handling. 
They  are  somewhat  expensive  and  if  overheated  or  heated  too 
long  become  carbonized  and  brittle.  In  cold  weather  they  do 
not  stick  to  the  pipe  well  unless  the  pipe  is  heated  before  the 
joint  is  poured.  On  some  work  joints  have  been  poured  under 
water  with  these  compounds,  but  success  is  doubtful  without 
skillful  handling.  An  overheated  compound  will  make  steam 
in  the  joint  causing  explosions  which  will  blow  the  joint  clean, 


310 


CONSTRUCTION 


and  an  underheated  compound  will  harden  before  the  joint  is 
completed. 

The  materials  should  be  heated  in  an  iron  kettle  over  a  gaso- 
line furnace  or  other  controllable  fire,  until  they  just  commence 
to  bubble  and  are  of  the  consistency  of  a  thin  sirup.  Only  a 
sufficient  quantity  of  material  for  immediate  use  should  be  pre- 
pared and  it  should  be  used  within  10  to  15  minutes  after  it  has 
become  properly  heated.  The  ladle  used  should  be  large  enough 
to  pour  the  entire  joint  without  refilling.  There  are  other 
important  points  to  be  considered  in  pouring  joints  which  can  be 
learned  best  by  experience. 

The  quantity  of  material  necessary  for  making  these  joints, 
as  announced  by  the  manufacturers,  is  shown  in  Table  65. 

TABLE  65 
QUANTITY  OF  COMPOUND  NEEDED  FOR  POURED  JOINTS 


Quantity  of  Material  in  Pounds  per  Joint 

Diameter 

of  Pipe, 

Standard  Socket 

Deep  and  Wide  Socket 

in  Inches 

Jointite 

Filtite 

O.K. 

Jointite 

Filtite 

O.K. 

6 

0.82 

0.72 

0.42 

1.46 

1.28 

0.72 

8 

1.06 

0.95 

0.73 

1.82 

1.60 

1.25 

10 

1.30 

1.15 

0.89 

2.26 

1.98 

1.52 

12 

2.08 

1.82 

1.42 

2.65 

2.32 

1.80 

15 

2.52 

2.20 

1.74 

3.20 

2.80 

2.20 

18 

3.02 

2.64 

2.58 

3.75 

3.29 

3.25 

20 

3.44 

3.00 

2.86 

4.30 

3.78 

3.60 

22 

3.62 

3.16 

3.13 

4.62 

4.07 

3.97 

24 

4.03 

3.50 

3.41 

4.91 

4.31 

4.27 

In  making  a  poured  joint  the  pipes  are  first  lined  up  in  posi- 
tion. A  hemp  or  oakum  gasket  is  forced  into  the  joint  to  fill  a 
space  of  about  f  of  an  inch.  An  asbestos  or  other  non-combustible 
gasket  such  as  a  rubber  hose  smeared  with  clay  is  forced  about 
J  inch  into  the  opening  between  the  bell  and  the  spigot  and  the 
compound  is  poured  down  one  side  of  the  pipe  through  a  hole 
broken  in  the  bell,  until  it  appears  on  the  other  side,  and  the  hole 


THE  INVERT  311 

/ 

is  filled.  Occasionally  the  non-combustible  gasket  is  wrapped 
tightly  around  the  spigot  of  the  pipe  and  pressed  or  tied  firmly 
to  the  bell.  In  pouring  cement  grout  joints  a  paper  gasket 
is  used  which  is  held  to  the  bell  and  spigot  by  draw  strings. 
Greater  speed  in  construction  and  economy  in  the  use  of  materials 
are  obtained  by  joining  two  or  three  lengths  of  pipe  on  the  bank 
and  lowering  them  into  the  trench  as  a  unit.  The  pipes  are  set  in 
a  vertical  position  on  the  bank  with  the  bell  end  up,  one  length 
resting  in  the  other.  The  joint  is  calked  with  hemp  and  poured 
without  the  use  of  the  gasket.  The  joint  should  always  be  poured 
immediately  after  being  calked  so  that  the  hemp  can  not  become 
water  soaked.  The  asbestos  gasket  should  be  removed  as  soon 
as  possible  after  the  joint  is  poured  in  order  to  prevent  sticking 
with  resultant  danger  of  breaking  of  the  joint  when  attempting 
to  pull  the  gasket  free. 

One  man  can  pour  about  33  eight-inch  joints,  and  two  men 
can  complete  about  26  twelve-inch  joints  per  hour  on  the  bank 
where  conditions  are  more  or  less  fixed. 

182.  Labor  and  Progress. — The  labor  required  for  the  laying 
of  pipe  sewers,  exclusive  of  excavation,  bracing  and  backfilling, 
consists  of  pipe  layers  and  helpers.     For  pipes  24  to  27  inches 
in  diameter  or  smaller  one  pipe  layer  and  one  or  more  helpers 
are  necessary,  dependent  on  the  size  of  the  pipe  and  the  depth 
of  the  trench.     For  larger  pipes  two  pipe  layers  can  work  econom- 
ically each  working  on  one-half  of  the  pipe  and  making  half  of 
the  joint.     The  speed  of  pipe  laying  is  ordinarily  limited  by  the 
speed  of  the  excavation,  but  on  a  job  in  Topeka,  Kan.,1  where 
the  average  day's  progress  with  a  machine  excavator  was  200 
to  500  feet  of  trench  per  day,  the  pace  was  limited  by  the  speed 
of  the  pipe  laying  gang.     This  gang  consisted  of  two  pipe  layers 
in  the  trench  and  two  helpers  on  the  surface.     The  sizes  of  pipes 
handled  were  from  8  to  27  inches. 

BRICK  AND  BLOCK  SEWERS 

183.  The   Invert. — In  good   firm   ground   the   excavation  is 
cut  to  the  shape  of  the  sewer  and  the  bricks  are  laid  directly  on 
the  ground,  being  embedded  in  a  thick  layer  of  mortar.     After 
the  foundation  has  been  prepared  and  before  the  bricks  are  laid, 

1  Eng.  News,  Vol.  75,  1916,  p.  592. 


312  CONSTRUCTION 

two  wooden  templates,  called  profiles,  are  prepared,  similar  to  that 
shown  in  Fig.  126,  to  conform  to  the  shape  of  the  inside  and 
outside  of  the  sewer.  Each  course  of  bricks  is  represented  by 
a  row  of  nails  in  the  profile  and  each  nail  corresponds  to  a  joint 
in  the  row.  The  two  profiles  are  set  true  to  line  and  grade.  A 
cord  is  stretched  tightly  between  the  two  lowest  nails  on  opposite 
templates  and  a  row  of  bricks  is  laid.  The  bricks  are  laid 
radially  and  on  edge  with  their  long  dimension  parallel  to  the 
axis  of  the  sewer  and  with  one  edge  just  touching  the  string. 
As  each  one  or  two  or  three  rows  are  completed  the  guide  line  is 
moved  up  to  the  next  nails.  When  the  bricks  are  laid  on  the 
ground  all  but  large  depressions  are  filled  in  with  tamped  sand  or 
mortar  by  the  masons.  Approximately  the 
same  number  of  rows  of  bricks  is  kept  com- 
pleted on  either  side  of  the  center  line.  The 

FIG.  126. Profile  for   succeeding  courses  follow  within  three  to  five 

Brick  Sewers.  rows  of  each  other,  the  only  bond  between 
courses  being  the  mortar  joint.  This  is 
called  row  lock  bond  and  with  few  exceptions  has  been  used  on 
all  brick  sewers  in  the  United  States.  As  the  sides  of  the  sewer 
become  higher  during  the  construction,  platforms  must  be  built 
for  the  masons.  These  platforms  are  built  of  wood  and  rest 
directly  on  the  green  brickwork.  They  should  be  designed  to 
spread  the  load  as  much  as  possible.  The  brickwork  of  the  invert 
is  continued  up  in  this  way  to  the  springing  line.  As  soon  as 
one  section  is  completed  one  profile  is  moved  10  to  20  feet  ahead 
along  the  trench  according  to  the  standard  length  of  sections, 
and  set  in  position.  The  line  is  then  strung  from  it  to  nails 
driven  or  pushed  into  the  cement  joints  of  the  last  completed 
section.  Between  work  done  on  separate  days  the  bricks  are 
racked  back  in  courses  to  provide  a  satisfactory  bond. 

In  ground  too  soft  to  support  the  brickwork  directly  a  cradle 
is  prepared  by  placing  profiles  in  position  in  the  sewer  and  nailing 
2-inch  planks  to  these  profiles,  first  firmly  tamping  earth  under 
the  planks.  The  bricks  are  laid  in  this  cradle  in  a  manner 
similar  to  that  explained  for  sewers  with  a  firm  foundation.  In 
still  softer  ground  it  may  be  necessary  to  construct  a  concrete 
cradle  to  support  the  bricks. 

184.  The  Arch. — The  arch  centering  consists  of  a  wooden 
form  made  up  of  wooden  ribs  as  shown  in  Fig.  127.  The  center 


BLOCK  SEWERS  313 

conforms  to  the  shape  of  the  inside  of  the  arch  with  allowance 
for  the  thickness  of  the  lagging.  The  lagging  is  nailed  on  the  ribs 
in  straight  strips  parallel  to  the  axis  of  the  sewer.  The  center 
is  supported  on  triangular  struts  resting  against  the  sides  and  on 
the  bottom  of  the  sewer  and  is  lifted  into  position  by  wedges 
driven  between  it  and  the  support.  The  centers  may  be  placed 
immediately  after  the  completion  of  the  invert,  or  a  day  or  two 
may  be  allowed  to  pass  to  give  the  invert  an  opportunity  to  set. 
After  the  centers  are  fixed  in  place  the  arch  brick  are  carried  up 
evenly  on  each  side  and  are  pounded  firmly  into  place.  The  center 
is  usually,  but  not  always 
"  struck  "  immediately,  and 
the  arch  brick  are  cleaned 
and  pointed  up  from  the 
inside.  The  outside  is  cov- 
ered with  a  layer  of  J  to  f 
of  an  inch  of  cement  mortar 
and  may  be  backfilled  to  the 
top  of  the  arch  in  order  to 
maintain  the  moisture  of  the  FIG.  127.  -Centering  for  Brick  Sewer, 
mortar  during  setting  and  to 

press  the  bricks  of  the  arch  together  firmly.  The  centers  are  some- 
times made  collapsible  so  that  they  can  be  carried  or  rolled  through 
the  finished  brickwork  to  the  advanced  position.  In  "  striking  " 
the  centers  the  wedges  are  removed  and  the  wings  folded  in. 

In  tunneling,  the  invert  of  the  sewer  is  constructed  in  the  same 
fashion  as  for  open-cut  work.  The  arch  centering  is  made  in 
short  sections  and  the  bricks  are  put  in  position  by  reaching  in 
over  the  end  of  the  centering.  All  of  the  timbering  of  the  tunnel 
is  removed  except  the  poling  boards  or  lagging  against  which 
the  bricks  or  mortar  are  tightly  pressed,  the  boards  being  bricked 
in  permanently. 

185.  Block  Sewe'rs. — Sewers  made  of  unit  blocks  of  concrete 
or  vitrified  clay  are  constructed  in  a  similar  manner  to  brick 
sewers.  Fig.  128  shows  the  construction  of  a  block  sewer  at 
Clinton,  Iowa.  In  this  sewer  there  are  two  rings;  an  inside  one  of 
solid  blocks  and  an  outside  one  of  hollow  blocks.  Block  sewers 
do  not  demand  the  skill  in  construction  that  is  demanded  by 
brick  sewers,  as  the  blocks  are  so  cast  that  the  joints  are  radial, 
whereas  only  experienced  masons  can  lay  bricks  radially. 


314 


CONSTRUCTION 


186.  Organization. — The  number  of  men  employed  on  a 
brick  or  block  sewer  is  proportioned  according  to  the  size  of  the 
sewer  and  the  working  conditions.  The  number  of  men  working 
on  different  tasks  usually  bears  the  same  ratio  to  the  number 
of  masons  employed,  regardless  of  the  size  of  the  work.  These 

proportions  are  shown 
for  different  jobs,  in 
Table  66. 

187.  Rate  of  Progress. 
— In  a  general  way  it  can 
be  assumed  that  the  lay- 
ing of  1,000  bricks  will 
require  3J  hours  of  the 
time  of  one  mason,  10 
man  hours  for  helpers 
and  laborers,  2  barrels  of 
cement,  0.6  cubic  yard  of 
sand,  and  about  10  feet 
board  measure  of  center- 
ing. One  thousand  bricks 
will  make  about  2  cubic 
yards  of  brickwork.  To 
the  costs,  as  estimated 
on  the  basis  of  materials 
and  labor,  must  be  added 
about  15  per  cent  for 
overhead  and  an  addi- 
tional amount  for  the 
contractor's  profit.  The  number  of  bricks  required  in  various 
size  sewers  is  shown  in  Table  67.  A  mason  can  lay  more  bricks 
per  hour  in  a  large  sewer  than  in  a  small  one  as  there  is  a  smaller 
percentage  of  face  work,  there  is  more  room  to  work,  and  it  is 
easier  to  lay  the  bricks  radially.  The  number  of  bricks  laid  and 
the  rate  of  progress  on  various  jobs  are  shown  in  Table  68. 


FIG.  128. — Segmental  Block  Sewer  at  Clinton, 
Iowa, 


CONCRETE  SEWERS 

188.  Construction  in  Open  Cut. — In  the  construction  of  sewer 
pipe  of  cement  and  concrete  one  of  two  methods  may  be  em- 
ployed; 1st,  to  manufacture  the  pipe  in  a  plant  at  some  distance 


CONSTRUCTION  IN  OPEN  CUT 


313 


from  the  place  of  final  use,  or  2nd,  to  manufacture  the  pipe  in 
place.  The  methods  of  the  manufacture  of  cement  and  concrete 
pipe  which  are  to  be  transported  to  the  place  of  use  are  treated 
in  Chapter  VIII.  The  process  of  constructing  the  pipes  in  place 
is  ordinarily  used  for  pipes  48  inches  or  more  in  diameter.  For 
smaller  sizes,  brick,  vitrified  clay,  and  precast  cement  pipes  are 
usually  more  economical. 

TABLE  66 

ORGANIZATIONS   FOR  THE   CONSTRUCTION   OF  BRICK  AND   BLOCK  SEWERS 


Type  of  Work 

General 
Ratio  on 
Basis  of 
Four  Brick 
Layers 

15-foot, 
5-ring 
Brick, 
Chicago 

66-inch 
Circular 
Brick, 
Gary 

84-inch 
Circular 
Brick, 
Gary 

84-  to 
108-inch 
Sewer 
Brick  in 
Detroit 
Tunnel 

42-inch 
Lock- 
Joint 
Tile 
Block 

Foreman  

1 

1 

1 

1 

1 

1 

Brick  layers  .... 

4 

12 

6 

6 

5 

2 

Helpers  

2 

11 

3 

3 

1 

Scaffold  men  .... 

2 

21 

3 

Brick  tossers  .... 

2 

7 

15 

2 

Brick  carriers  .  .  . 

2 

2 

2 

Cement  mixers  .  . 

2 

6 

6 

5 

1 

Cement  carriers  . 

2 

10 

8 

Form  setters  .... 

1 

3 

3 

Laborers  

1 

8 

19 

3 

14 

7 

Source  of 
Information  1 

Municipal 
Engineering, 
Vol.54,p.228 

H.  P. 

Gillette,  Handbook  of  Cost  Data 

The  preparation  of  the  foundation  of  a  concrete  sewer  is 
similar  to  that  for  a  brick  sewer.  If  the  ground  is  suitable  the 
trench  is  shaped  to  the  outside  form  of  the  sewer  and  the  con- 
crete poured  directly  on  it.  In  soft  material  which  would  give 
poor  support  to  a  sewer  with  a  rounded  exterior,  the  bottom  of 
the  trench  is  cut  horizontal  and  a  concrete  cradle  of  poorer  quality 
than  that  in  the  finished  sewer  is  poured  on  the  soft  ground,  on  a 
board  platform,  on  piles,  or  on  cribbing  supported  on  piles. 

-If  the  invert  of  the  sewer  is  so  flat  that  the  concrete  will 
stand  without  an  inside  form  the  shape  of  the  invert  is  obtained 


316 


CONSTRUCTION 


by  a  screed  or  straight-edge  which  is  passed  over  the  surface  of 
the  concrete  and  guided  on  two  centers,  or  on  one  center  and  the 
face  of  the  finished  work.  The  construction  of  a  flat  invert 
sewer  at  Baltimore  is  shown  in  Fig.  1.  The  center  for  the  con- 
crete is  shown  in  the  foreground.  When  the  concrete  for  the 
next  section  is  poured  it  will  be  smoothed  to  shape  by  a  screed  or 
straight-edge  resting  on  the  face  of  the  finished  concrete  and  the 
center.  The  center  is  shaped  to  conform  to  that  of  the  finished 
concrete.  It  is  firmly  staked  in  position  and  acts  as  a  bulk- 
head for  the  concrete  as  it  is  poured,  as  well  as  a  guide  for  the 
screed. 

TABLE  67 

BRICK  MASONRY  IN  CIRCULAR  SEWERS.     CUBIC  YARDS  PER  LINEAR  FOOT 
(From  H.  P.  Gillette) 


Diameter, 
Feet  and  Inches 

One  Ring 
(4£  Inches) 

Two  Ring 

(9  Inches) 

Three  Ring 
(13  1  Inches) 

2 

0 

0.103 

0.240 

2 

6 

0.125 

0.280 

3 

0 

0.147 

0.327 

3 

6 

0.169 

0.371 

4 

0 

0.191 

0.415 

4 

6 

0.213 

0.458 

5 

0 

0.234 

0.501 

0.802 

5 

6 

0.256 

0.545 

0.867 

6 

0 

0.278 

0.589 

0.933 

6 

6 

0.633 

1.000 

7 

0 

0.677 

1.063 

7 

6 

0.720 

1.128 

8 

0 

0.763 

1.193 

8 

6 



0.807 

1.260 

9 

0 

0.851 

1.325 

9 

6 

0.895 

1.390 

10 

0 

0.938 

1.456 

If  inside  forms  are  to  be  used  they  ape  made  as  units  in  lengths 
of  12  or  16  feet  for  wooden  forms,  and  5  feet  for  steel  forms. 
The  inside  form  is  supported  by  precast  concrete  blocks  placed 
under  it  and  which  are  concreted  into  the  sewer.  It  is  held  in 
position  by  cleats  nailed  to  the  outside  form,  to  the  sheeting,  or 


RATE  OF  PROGRESS 


317 


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318 


CONSTRUCTION 


wedged  against  the  outside  of  the  trench.  In  some  cases, 
particularly  where  steel  forms  are  used,  the  inside  form  is  hung 
by  chains  from  braces  across  the  trench  as  is  shown  in  Fig.  129. 
The  form  is  easily  brought  to  proper  grade  by  adjustment  of  the 
turnbuckles  and  is  then  wedged  into  position  to  prevent  move- 
ment either  sideways  or  upwards  during  the  pouring  of  the 
concrete.  It  may  be  necessary  to  weight  the  forms  down  to 
prevent  flotation.  Cross  bracing  in  the  trench  which  interferes 
with  the  placing  of  the  form  is  removed  and  the  braces  are  placed 


FIG.  129.— Blaw  Standard  Half-Round  Sewer  Form,  Suspended  from  Overhead 

Support. 
Courtesy,  Blaw  Steel  Form  Co. 

against  the  form  until  the  concrete  is  poured.    They  are  removed 
immediately  in  advance  of  the  rising  concrete. 

The  sewer  section  may  be  built  as  a  monolith,  in  two  parts,  or 
in  three  parts.  In  casting  the  sewer  as  a  monolith  the  complete 
full  round  inside  form  is  fixed  in  place  by  concrete  blocks  and 
wires.  The  full  round  outside  form  is  completed  as  far  as  pos- 
sible without  interfering  too  much  with  the  placing  and  tamping 
of  the  concrete.  The  concrete  is  poured  from  the  top,  being 
kept  at  the  same  height  on  each  side  of  the  form,  and  tamped 
while  being  poured.  The  remaining  panels  of  the  outside  form 
are  placed  in  position  as  the  concrete  rises  to  them.  An  open- 
ing is  left  at  the  top  of  the  outside  arch  forms  which  is  of  such  a 


CONSTRUCTION  IN  OPEN  CUT  319 

width  that  the  concrete  will  stand  without  support.  The  casting 
of  sewers  as  a  monolith  is  difficult  and  is  usually  undesirable  be- 
cause of  the  uncertainty  of  the  quality  of  the  work.  It  has  the 
advantage,  however,  of  eliminating  longitudinal  working  joints 
in  the  sewers  which  may  allow  the  entrance  of  water  or  act  as  a 
line  of  weakness. 

If  the  sewer  is  to  be  cast  in  two  sections  the  invert  is  poured 
to  the  springing  line  or  higher.  A  triangular  or  rectangular 
timber  is  set  in  the  top  of  the 
wet  concrete  as  shown  in  Fig.  /  -  j  j — c — c— '  i  '  •% 


130.      When  the  concrete  has  set 

the  timber  is  removed  and  the 

groove  thus   left   forms   a  work-    FIG.  130.— Construction  Joints  for 

ing   joint  with  the   arch.     After  Concrete  Sewers. 

the  invert   concrete  has  set,  the 

arch  centering  is  placed  and  the  arch  is  completed.     This  is  the 

most  common    method   for   the    construction   of  medium-sized 

circular  sewers. 

Large  sewers  with  relatively  flat  bottoms  are  poured  in  two  or 
three  sections.  First  the  invert  is  poured  without  forms  and  is 
shaped  with  a  screed.  About  6  inches  of  vertical  wall  is  poured 
at  the  same  time.  This  acts  as  a  support  for  the  side-wall  forms. 
The  side  walls  reach  to  the  springing  line  of  the  arch  and  are 
poured  after  the  invert  has  set.  At  the  third  pouring  the  arch 
is  completed.  The  sewer  shown  in  Fig.  1  is  being  poured  in  two 
steps,  as  the  side  walls  are  so  low  that  they  are  poured  at  the 
same  time  as  the  invert.  A  transverse  working  joint  similar  to 
one  of  the  types  used  in  Fig.  130  is  set  between  each  day's  work. 

The  length  of  the  form  used  and  the  capacity  of  the  plant 
should  be  adjusted  so  that  one  complete  unit  of  invert,  side  wall, 
or  arch  can  be  poured  in  one  operation.  The  forms  are  left  in 
place  until  the  concrete  has  set.  Invert  and  side  wall-forms  are 
generally  left  in  position  for  at  least  two  days,  and  in  cold  weather 
longer.  The  arch  forms  are  left  in  place  for  double  this  time. 
For  example  if  20  feet  of  invert  and  arch  can  be  poured  in  a  day, 
60  feet  of  invert  form  and  100  feet  of  arch  form  will  be  required. 
As  the  forms  are  released  they  must  be  moved  forward  through 
those  in  place.  For  this  reason  collapsible  or  demountable 
forms  are  necessary  and  steel  forms  are  advantageous.  Wooden 
arch  forms  are  sometimes  dismantled  and  carried  forward  in 


320 


CONSTRUCTION 


sections,  but  are  preferably  designed  to  collapse  as  shown  in 
Fig.  131,  so  that  they  can  be  pulled  through  on  rollers  or  a  carriage. 
189.  Construction  in  Tunnels. — In  tunnels  the  invert  and 
side  walls  are  constructed  in  the  same  manner  as  for  open  cut 
work.  The  tunneling,  which  acts  as  the  outside  form,  is 
concreted  permanently  in  place.  The  concreting  of  a  tunnel 
by  hand  is  shown  in  Fig.  132.  If  the  work  is  to  be  done  by  hand 
the  concrete  is  thrown  in  between  the  ribs  of  the  arch  centering 
and  behind  the  plates  or  lagging,  which  are  set  in  advance  of  the 
rising  concrete.  The  lagging  plates  are  5  feet  long  which  makes 


,'l  x3  Lagging 
Covered  with 
Galvanized 
Sheet  Iron. 


FIG.  131. — Section  through  a  Collapsible  Wood  Form. 

it  possible  to  throw  the  concrete  in  place  at  the  arch,  and  to  tamp 
it  in  place  from  the  end.  A  bulkhead  and  a  well-greased  joint 
timber  are  placed  in  position  as  the  concrete  rises. 

Pneumatic  transmission  of  concrete  is  also  used  for  filling 
the  arch  forms  as  well  as  the  side  walls  and  invert  forms.  In 
using  this  method  the  mixer  may  be  placed  at  the  surface  or  at 
the  bottom  of  the  shaft  or  other  convenient  permanent  location 
which  may  be  some  distance  from  the  form.  The  mixture  is 
discharged  into  a  pipe  line  through  which  it  is  blown  by  air  to 
the  forms.  The  starting  pressure  of  about  80  pounds  per 
square  inch  can  be  reduced  after  flow  has  commenced.  In  con- 
structing the  St.  Louis  Water  Works  tunnel  the  compressor 
equipment  for  moving  the  concrete  had  a  capacity  of  1,600 


MATERIALS  FOR  FORMS  321 

cubic  feet  per  minute  at  a  pressure  of  110  pounds.  The  tunnel 
is  horse-shoe  shaped,  8  feet  in  height  and  with  walls  varying  from 
9  to  20  inches  in  thickness.  The  extreme  travel  of  the  concrete 
was  1,100  feet  in  an  8  inch  pipe.  The  amount  of  air  consumed  at 
110  pounds  varied  from  1.2  to  1.7  cubic  feet  of  free  air  per  linear 
foot  of  pipe.  By  the  time  the  batch  had  been  discharged  the 
pressure  had  reduced  to  25  to  40  pounds,  depending  on  the  length 
of  the  pipe.  It  is  reported  that  a  6-inch  pipe  line  would  probably 
have  given  better  results. 


FIG.  132. — Ogier's  Run  Intercepting  Storm-Water  Drain,  Baltimore,  Mary- 
land. 

Placing  concrete  in  Arch.  T^e  steel  lagging  of  the  forms  is  carried  up  in  sections  as  the 
concrete  is  deposited.  The  drain  is  horse-shoe  shaped,  and  is  12  feet  3  inches  high  and  12 
feet  3  inches  wide. 


The  end  of  the  concrete  conveying  pipe  is  provided  with  a 
flexible  joint  the  simplest  form  of  which  can  be  made  by  slipping 
a  section  of  pipe  of  larger  diameter  over  the  end  of  the  trans- 
mission line.  The  concrete  is  deposited  directly  on  the  invert 
or  into  the  side-wall  forms  and  can  be  blown  into  the  arch  forms 
for  20  to  25  feet. 

190.  Materials  for  Forms. — The  materials  used  in  forms  for 
concrete  sewers  are:  wood,  wood  with  steel  lining,  and  steel 
alone.  The  first  cost  of  wood  forms  is  lower  than  that  of  steel 
but  their  life  is  relatively  short.  If  the  forms  are  to  be  used 
a  number  of  times  steel  is  more  economical.  With  proper  care 


322  CONSTRUCTION 

and  repairs  steel  forms  will  outlast  any  other  material.  Because 
of  the  increasing  price  of  lumber  and  improvements  in  steel 
forms,  wood  forms  are  not  frequently  used.  A  common  type 
of  specification  under  which  forms  are  used  is: 

The  material  of  the  forms  shall  be  of  sufficient  thick- 
ness and  the  frames  holding  the  forms  shall  be  of  sufficient 
strength  so  that  the  forms  shall  be  unyielding  during  the 
process  of  filling.  The  face  of  the  form  next  to  the  concrete 
shall  be  smooth.  If  wooden  forms  are  used  the  planking 
forming  the  lining  shall  invariably  be  fastened  to  the  stud- 
ding in  horizontal  lines,  the  ends  of  these  planks  shall  be 
neatly  butted  against  each  other,  and  the  inner  surface  of 
the  form  shall  be  as  nearly  as  possible  perfectly  smooth, 
without  crevices  or  offsets  between  the  ends  of  adjacent 
planks.  Where  forms  are  used  a  second  time,  they  shall 
be  freshly  jointed  so  as  to  make  a  perfectly  smooth  finish 
to  the  concrete.  All  forms  shall  be  water-tight  and  shall 
be  wetted  before  using. 

Any  material  in  contact  with  wet  concrete  should  be  oiled  or 
greased  beforehand  in  order  to  prevent  adherence  to  the  concrete. 

191.  Design  of  Forms.  —  The  design  of  forms  for  reinforced 
concrete  work  requires  some  knowledge  of  the  strength  of  materials 
and  the  theories  of  beams,  columns,  and  arches.  Forms  can  be 
constructed  without  such  knowledge  but  that  they  will  be  both 
economical  and  adequate  is  an  improbability.  The  ordinary 
beam  and  column  formulas  are  applicable  to  the  design  of  forms. 
The  maximum  bending  moment  for  sheeting  and  ribs  is  taken  as 

wl2 
—  ,  where  w  is  the  load  per  unit  length,  and  I  is  the  length  between 

o 

supports.     Sanford  Thompson  recommends  that  the  deflection  be 

wl? 
calculated  as  -,OOFT,  in  which  E  is  the  modulus  of  elasticity  of 


the  material,  and  /  is  the  moment  of  inertia  of  the  cross-section 
referred  to  the  neutral  axis.  The  horizontal  pressure  of  the  con- 
crete against  the  forms  has  been  expressed  empirically  by  E.  B. 
Smith,1  as 


1  Pressure  of  Concrete  on  Forms  Measured  in  Tests,  by  E.  B.  Smith, 
before  Am.  Concrete  Institute,  Feb.  15,  1920.  Abstracted  in  Eng.  News- 
Record,  Vol.  84,  1920,  p.  665. 


DESIGN  OF  FORMS  323 

in  which  P  =  lateral  pressure  in  pounds  per  square  inch; 
R  =  rate  of  filling  forms  in  feet  per  hour; 
#  =  head  of  fill.     Ordinarily  taken  as  %R,  but  in  cold 

weather  or  when  continuously  agitated  it  may  be 

as  high  as  \R] 

C  =  ratio,  by  volume,  of  cement  to  aggregate; 
S  =  consistency  in  inches  of  slump. 

Earlier  investigators  have  usually  concluded  that  the  pressures 
were  equal  to  those  caused  by  a  liquid  weighing  144  pounds 
per  cubic  foot,  but  the  tests  of  the  United  States  Bureau  of 
Public  Roads,  from  which  the  above  formula  was  devised,  show 
the  pressures  to  be  decidedly  below  this  amount  under  certain 
conditions. 

With  these  units  and  formulas  the  design  of  the  lagging  becomes 
a  matter  of  substitution  in,  and  the  solution  of,  the  equations 
produced.1  The  forces  acting  on  the  ribs  are  indeterminate. 
No  more  satisfactory  design  can  be  made 
for  the  ribs  than  to  follow  successful  prac- 
tice, or  what  is  seldom  done,  to  determine 
the  stresses  in  the  forms  by  the  application 
of  one  of  the  theories  for  the  solution  of 
arch  stresses.  The  sizes  of  the  lumber 
used  in  the  ribs  varies  from  1^X6  inches 
to  2X10  inches,  depending  on  the  size 
of  the  sewer.  If  vertical  posts  are  used 
at  the  ends  to  support  the  arch  forms  FlG  133.— Centering  for 
they  are  computed  as  columns  taking  the  Large  Forms, 

full  weight  of   the   arch.     If   the   span  is 

so  wide  that  radial  supports  are  used  as  shown  in  Fig.  133 
the  load  at  the  center  is  assumed  as  one-fourth  of  the  weight  of 
the  arch. 

192.  Wooden  Forms. — Norway  and  Southern  pine,  spruce, 
and  fir  are  satisfactory  for  form  construction.  White  pine  is 
satisfactory  but  is  generally  too  expensive.  The  hard  woods 
are  too  difficult  to  work.  The  lumber  should  be  only  partly 
dried  as  kiln-dried  lumber  swells  too  much  when  it  is  moistened, 
warping  the  forms  out  of  shape  or  crushing  the  lagging  at  the 

1  See,  also,  Concrete  Form  Design,  by  E.  F.  Rockwood,  Eng.  and  Con- 
tracting, Vol.  55,  1921,  p.  528. 


324 


CONSTRUCTION 


joints.  Green  lumber  must  be  kept  moist  constantly  to  prevent 
warping  before  use  and  when  it  is  used  it  does  not  swell  enough 
to  close  the  cracks.  The  lumber  should  be  dressed  on  the  face 
next  to  the  concrete  and  at  the  ends.  Either  beveled  or  matched 
lumber  may  be  used  for  lagging.  The  joint  made  by  beveled 


FIG.  134.  FIG.  135.  FIG.  136. 

FIG.  134. — Beveled  Joint  for  Wood  Fords. 

FIG.  135. — Collapsible  Wooden  Invert  Form  for  Concrete  Sewers. 
FIG.  136. — Support  for  Arch  Centering. 


<Joint. 


k  Nut 


FIG.  137. — Wooden  Forms  Used  in  Tunnel,  North  Shore  Sewer,  Sanitary  Dis- 
trict of  Chicago. 

Journal  Western  Society  of  Engineers,  Vol.  22,  p.  385. 

lumber  shown  in  Fig.  134  is  cheaper  but  less  satisfactory  than  a 
tongued  and  grooved  joint. 

Types  of  wooden  forms  are  shown  in  Figs.  135  and  136  for 
use  in  sewers  to  be  built  as  monoliths  or  in  two  portions.  Fig. 
137  shows  the  details  of  a  built-up  wooden  form  used  in  tunnel 
work  for  a  42|  inch  egg-shaped  sewer. 


STEEL  FORMS 


325 


193.  Steel-lined  Wooden  Forms. — Sheet  metal  linings  are 
sometimes  used  on  wooden  forms.  They  permit  the  use  of 
cheaper  undressed  lumber,  demand  less  care  in  the  joining  of  the 
lagging,  and  when  in  good  condition  give  a  smooth  surface  to 
the  finished  concrete.  Their  use  has  frequently  been  found 
unsatisfactory  and  more  expensive  than  well-constructed  wooden 
forms  because  of  the  difficulty  of  preventing  warping  and 
crinkling  of  the  metal  lining  and  in  keeping  the  ends  fastened 
down  so  that  they  will  not  curl.  Sheet  steel  or  iron  of  No.  18  or 
20  gage  (0.05  to  0.0375  of  an  inch)  weighing  2  to  1J  pounds 
per  square  foot  is  ordinarily  used  for  the  lining . 


FIG.  138.— Blaw  Standard    Full  Round  Telescopic  Sewer  Forms,  Showing 
Knocked-Down  Sections  Loaded  on  a  Truck. 
Courtesy,  Blaw  Steel  Form  Co. 

194.  Steel  Forms. — These  are  simple,  light,  durable,  and  easy 
to  handle.  The  engineer  is  seldom  called  upon  to  design  these 
forms  as  the  types  most  frequently  used  are  manufactured  by  the 
patentees  and  are  furnished  to  the  contractor  at  a  fixed  rental 
per  foot  of  form,  exclusive  of  freight  and  hauling  from  the  point 
of  manufacture.  The  forms  can  be  made  in  any  shape  desired, 
the  ordinary  stock  shapes  such  as  the  circular  forms  being  the 
least  expensive.  The  smaller  circular  forms  are  adjustable 
within  about  3  inches  to  different  diameters  so  that  the  same 
form  can  be  used  for  two  sizes  of  sewers.  The  same  form  can  be 
used  for  arch  and  invert  in  circular  sewers.  Fig.  138  shows  the 


326 


CONSTRUCTION 


collapsible  circular  forms  and  the  manner  in  which  they  are 
pulled  through  those  still  in  position.  Fig.  129  shows  a  half 
round  steel  form  swung  in  position  by  chains  and  turnbuckles 
from  the  trench  bracing,  and  Fig.  139  shows  the  free  unobstructed 
working  space  in  the  interior  of  some  large  steel  forms. 

195.  Reinforcement. — It  is  essential  that  the  reinforcement 
be  held  firmly  in  place  during  the  pouring  of  the  concrete.  A 
section  of  reinforcement  misplaced  during  construction  may 
serve  no  useful  purpose  and  result  in  the  collapse  of  the  sewer. 


FIG.  139. — Interior  of  .Steel  Forms  for  Calumet  Sewer,  Chicago. 

Sewer  is  16  feet  wide.     Note  absence  of  obstructions.     Courtesy,  Hydraulic  Steelcraft  Co. 

In  sewer  construction  a  few  longitudinal  bars  may  be  laid  in 
order  that  the  transverse  bars  may  be  wired  to  them  and  held 
in  position  by  notches  in  the  centering  and  in  fastenings  to  bars 
protruding  from  the  finished  work.  This  construction  is  shown 
in  Fig.  1.  The  network  of  reinforcement  is  held  up  from  the 
bottom  of  the  trench  by  notched  boards  which  are  removed  as 
the  concrete  reaches  them,  or  better  by  stones  or  concrete 
blocks  which  are  concreted  in.  Sometimes  the  reinforcement 
is  laid  on  top  of  the  freshly  poured  portion  of  the  concrete  the 
surface  of  which  is  at  the  proper  distance  from  the  finished  face 


COSTS  OF  CONCRETE  SEWERS  327 

of  the  work.  This  method  has  the  advantage  of  not  requiring 
any  special  support  for  the  reinforcement,  but  it  is  undesirable 
because  of  the  resulting  irregularity  in  the  reinforcement  spacing 
and  position. 

In  the  side  walls  the  position  of  the  reinforcement  is  fixed  by 
wires  or  metal  strips  which  are  fastened  to  the  outside  forms  or 
to  stakes  driven  into  the  ground.  Wires  are  then  fastened  to  the 
reinforcement  bars  and  are  drawn  through  holes  in  the  forms 
and  twisted  tight.  When  the  forms  are  removed  the  wires  or 
strips  are  cut  leaving  a  short  portion  protruding  from  the  face 
of  the  wall.  The  reinforcing  steel  from  the  invert  should  pro- 
trude into  the  arch  or  the  side  walls  for  a  distance  of  about  40 
diameters  in  order  to  provide  good  bond  between  the  sections. 
The  protruding  ends  are  used  as  fastenings  for  the  new  reinforce- 
ment. The  arch  steel  may  be  supported  above  the  forms  by 
specially  designed  metal  supports,  by  small  stones  or  concrete 
blocks  which  are  concreted  into  the  finished  work;  or  by  notched 
strips  of  wood  which  are  removed  as  the  concrete  approaches 
them.  Strips  of  wood  are  not  satisfactory  because  thay  are 
sometimes  carelessly  left  in  place  in  the  concrete  resulting  in  a 
line  of  weakness  in  the  structure.  Metal  chairs  are  the  most 
secure  supports.  They  are  fastened  to  the  forms  and  the  bars 
are  wired  to  the  chairs.  In  some  instances  the  entire  rein- 
forcement has  been  formed  of  one  or  two  bars  which  are  fastened 
into  position  as  a  complete  ring.  This  results  in  a  better  bond  in 
the  reinforcement,  requires  less  fastening  and  trouble  in  handling, 
but  is  in  the  way  during  the  pouring  of  the  concrete  and  inter- 
feres with  the  handling  of  the  forms. 

196.  Costs  of  Concrete  Sewers. — Under  present  day  conditions 
a  general  statement  of  the  costs  of  an  engineering  structure  can 
not  be  given  with  accuracy.  Only  the  items  of  labor,  materials, 
and  transportation  that  go  to  make  up  the  cost  can  be  estimated 
quantitively,  and  the  total  cost  computed  by  multiplying  the 
amount  of  each  item  by  its  proper  unit  cost  obtained  from  the 
market  quotations. 

A  summary  of  some  of  the  items  that  go  to  make  up  the  cost 
of  a  concrete  sewer  and  the  relative  amount  of  these  items  on 
different  jobs  is  given  in  Tables  69  and  70. 


328 


CONSTRUCTION 


TABLE  69 

DIVISION  OF  LABOR  COSTS  FOR  THE  CONSTRUCTION  OF 
96-INCH  CIRCULAR  CONCRETE  SEWER 


Classification  of  Labor 

Classification  of  Work 

Task  or  Title 

Number 
of 
men 

Total 
dollars 
per  day 

Type  of  Work 

Dollars 
per  foot 

Superintendent  

1 
1 
1 
2 
10 
2 
1 
2 
2 
2 
3 
16 
3 
1 

6.00 
3.50 
2.00 
3.30 
16.50 
3.30 
3.00 
3.30 
3.30 
3.30 
5.25 
26.40 
5.25 
1.00 
5.00 

Excavation  

1.80 
0.58 
0.17 
0.45 
0.33 
1.17 
1.54 
0.29 

0.20 

0.09 
0.62 
0.62 

Hoister  (engineman)  

Bottom  plank 

Pulling  sheeting                       .  . 

Backfilling 

On  dump  cars  
Carpenter  on  bracing      

Making  and  placing  invert  .... 
Making  and  placing  arch  
Laying  brick  in  invert  
Bending  and  placing  steel  in 
arch  
Bending  and  placing  steel  in 
invert  
Moving  forms  and  centers  .... 
Watchmen,  water  boy,  etc.  .  .  . 

Total 

Carpenters'  helpers  
Laying  bottom  

Pulling  sheeting  
Mixing  and  placing  concrete. 
On  steel  forms  
Water  boy 

Coal  and  oil 

Total 

90.40 

7.86 

NOTES. — Trench  was  12j  feet  wide  and  of  various  depths.     At  depth  of  12  feet  the 
cost  of  excavation  was  $1.61  per  foot.     From  Engineering  and  Contracting,  Vol.  47,  p.  157. 

BACKFILLING 

197.  Methods. — Careful  backfilling  is  necessary  to  prevent  the 
displacement  of  the  newly  laid  pipe  and  to  avoid  subsequent 
settlement  at  the  surface  resulting  in  uneven  street  surfaces  and 
dangers  to  foundations  and  other  structures. 

The  backfilling  should  commence  as  soon  as  the  cement  in  the 
joints  or  in  the  sewer  has  obtained  its  initial  set.  Clay,  sand, 
rock  dust,  or  other  fine  compactible  material  is  then  packed  by 
hand  under  and  around  the  pipe  and  rammed  with  a  shovel  and 
light  tamper.  This  method  of  filling  is  continued  up  to  the  top 
of  the  pipe.  The  backfill  should  rise  evenly  on  both  sides  of  the 
pipe  and  tamping  should  be  continuous  during  the  placing  of  the 
backfill.  For  the  next  2  feet  of  depth  the  backfill  should  be  placed 
with  a  shovel  so  as  not  to  disturb  the  pipe,  and  should  be  tamped 
while  being  placed,  but  no  tamping  should  be  done  within  6 
inches  of  the  crown  of  the  sewer.  The  tamping  should  become 


BACKFILLING  METHODS 


329 


progressively   heavier   as   the   depth   of   the   backfill   increases. 
Generally  one  man  tamping  is  provided  for  each  man  shoveling. 

TABLE  70 

DIVISION  OF  COSTS  FOR  THE  CONSTRUCTION  OF  CONCRETE  SEWERS 

Gillette's  Handbook  of  Cost  Data. 


Item 

Location 

Fond 
du  Lac 

South 
Bend 

Wilming- 
ton 

Richmond,  Indiana 

Diameter  in  inches  
Shape     .                 

30 
circular 
plain 
0.11 
47 
1.20 

39.0* 
1.5 
12.4 
0.9 
0.7 
0.0 
23.0 
2.0 
12.5 
8.0 
8 
1908 

66 
circular 
rein. 
0.594 
24  to  36 
4.40 

33.5 
11.5 
15.5 

11.5 
20.0 

8.0 

10 
1906 

53 
horseshoe 
rein. 
0.37 

2.97 
33.0 
18.9 

14.5 
27.5 

6.1 

54                 48 
circular       circular 
rein.             rein. 
5"  shell       5"  shell 

1.35            1.08 
17.1 
19.3 

22.3 
32.  &t 

9.3 

Prewar  conditions 

42 
circular 
rein. 
4"  shell 

0.91 

Plain  or  reinforced  
Cubic  yards  per  foot.  .  .  . 
Daily  progress,  feet  
Cost  per  foot,  dollars  .... 
Per  cent  of  total  cost: 

Tools 

Sand  and  gravel  . 

Lumber                 

Water  
Reinforcing  
Cement  
Frost  prevention  
Forms  

Engineering  
Length  of  day,  hours  .... 
Year  of  construction  .... 

*  Includes  6  cents  per  foot  for  excavation, 
labor  cost. 

t  Cement  at  $1.25  per  barrel. 


Labor  for  this  was  58  per  cent  of  the  total 


Above  a  point  2  feet  above  the  top  of  the  sewer  the  method 
pursued  and  the  care  observed  in  backfilling  will  depend  on  the 
character  of  the  backfilling  material  and  the  location  of  the 
sewer.  If  the  sewer  is  in  a  paved  street  the  backfill  is  spread  in 
layers  6  inches  thick  and  tamped  with  rammers  weighing  about 
40  pounds  with  a  surface  of  about  30  square  inches.  One  man 
tamping  for  each  man  shoveling  is  frequently  specified.  If  no 
pavement  is  to  be  laid  but  it  is  required  that  the  finished  surface 
shall  be  smooth,  slightly  less  care  need  be  taken  and  only  one 
man  tamping  is  specified  for  each  two  men  shoveling.  On  paved 
streets  a  reinforced  concrete  slab  with  a  bearing  of  at  least  12 
inches  on  the  undisturbed  sides  of  the  trench  may  be  designed 


330  CONSTRUCTION 

to  support  the  pavement  and  its  loads.  This  is  of  great  help 
in  preventing  the  unsightly  appearance  and  roughness  due  to 
an  improperly  backfilled  trench.  On  unpaved  streets  the 
backfill  is  crowned  over  the  trench  to  a  depth  of  about  6  inches 
and  then  rolled  smooth  by  a  road  roller.  In  open  fields,  in  side 
ditches,  or  in  locations  where  obstruction  to  traffic  or  unsight- 
liness  need  not  be  considered,  after  the  first  2  feet  of  backfill  have 
been  placed  with  proper  care,  the  remainder  is  scraped  or  thrown 
into  the  trench  by  hand  or  machine,  care  being  taken  not  to 
drop  the  material  so  far  as  to  disturb  the  sewer. 

If  the  top  of  the  sewer,  manhole,  or  other  structure  comes 
close  to  or  above  the  surface  of  the  ground,  an  earth  embank- 
ment should  be  built  at  least  3  feet  thick  over  and  around  the 
structure.  The  embankment  should  have  side  slopes  of  at  least 
1J  on  1  and  should  be  tamped  to  a  smooth  and  even  finish. 

If  sheeting  is  to  be  withdrawn  from  the  trench  it  should  be 
withdrawn  immediately  ahead  of  the  backfilling,  and  in  trenches 
subject  to  caving  it  may  be  pulled  as  the  backfilling  rises. 

Puddling  is  a  process  of  backfilling  in  which  the  trench  is 
filled  with  water  before  the  filling  material  is  thrown  in.  It 
avoids  the  necessity  for  tamping  and  can  be  used  satisfactorily 
with  materials  that  will  drain  well  and  will  not  shrink  on  drying. 
Sand  and  gravel  are  suitable  materials  for  puddling,  heavy 
clay  is  unsatisfactory.  Puddling  should  not  be  resorted  to  before 
the  first  2  feet  of  backfill  has  been  carefully  placed.  More 
compact  work  can  be  obtained  by  tamping  than  with  puddling. 

Frozen  earth,  rubbish,  old  lumber,  and  similar  materials 
should  not  be  used  where  a  permanent  finished  surface  is  desired 
as  these  will  decompose  or  soften  resulting  in  settlement.  Rocks 
may  be  thrown  in  the  backfill  if  not  dropped  too  far  and  the 
earth  is  carefully  tamped  around  and  over  them.  In  rock 
trenches  fine  materials  such  as  loam,  clay,  sand,  etc.,  must  be 
provided  for  the  backfilling  of  the  first  portion  of  the  trench  for 
2  feet  over  the  top  of  the  pipe.  More  clay  can  generally  be  packed 
in  an  excavation  than  was  taken  out  of  it,  but  sand  and  gravel 
occupy  more  space  than  originally  even  when  carefully  tamped. 

Tamping  machines  have  not  come  into  general  use.  One 
type  of  machine  sometimes  used  consists  of  a  gasoline  engine 
which  raises  and  drops  a  weighted  rod.  The  rod  can  be  swung 
back  and  forth  across  the  trench  while  the  apparatus  is  being 


BACKFILLING  METHODS  331 

pushed  along.  It  is  claimed  that  two  men  operating  the  machine 
can  do  the  work  of  six  to  ten  men  tamping  by  hand.  The  machine 
delivers  50  to  60  blows  per  minute,  with  a  2  foot  drop  of  the  80 
to  90  pound  tamping  head. 

Backfilling  in  tunnels  is  usually  difficult  because  of  the  small 
space  available  in  which  to  work.  Ordinarily  the  timbering  is 
left  in  place  and  concrete  is  thrown  in  from  the  end  of  the  pipe 
between  the  outside  of  the  pipe  and  the  tunnel  walls  and  roof. 
If  vitrified  pipe  is  used  in  the  tunnel,  the  backfilling  is  done  with 
selected  clayey  material  which  is  packed  into  place  around  the 
pipe  by  workmen  with  long  tamping  tools.  The  backfilling 
should  be  done  with  care  under  the  supervision  of  a  vigilant 
inspector  in  order  that  subsequent  settlement  of  the  surface  may 
be  prevented. 


CHAPTER  XII 
MAINTENANCE  OF  SEWERS 

198.  Work  Involved. — The  principal  effort  in  maintaining 
sewers  is  to  keep  them  clean  and  unobstructed.  A  sewerage 
system,  although  buried,  cannot  be  forgotten  as  it  will  not  care 
for  itself,  but  becoming  clogged  will  force  itself  on  the  attention 
of  the  community.  Besides  the  cleaning  and  repairing  of 
sewers  and  the  making  of  inspections  for  determining  the  neces- 
sity for  this  work,  ordinances  should  be  prepared  and  enforced 
for  the  purpose  of  protecting  the  sewers  from  abuse.  Inspec- 
tions to  determine  the  amount  of  the  depreciation  of  sewers 
with  a  view  towards  possible  renewal,  or  to  determine  the 
capacity  of  a  sewer  in  relation  to  the  load  imposed  upon  it  are 
sometimes  necessary.  The  valuation  of  the  sewerage  system 
as  an  item  in  the  inventory  of  city  property  may  be  assigned  to 
the  engineer  in  charge  of  sewer  maintenance. 

The  work  involved  in  the  inspection  and  cleaning  of  sewers 
in  New  York  City  for  the  year  ending  May,  1914,  included  the 
removal  of  22,687  cubic  yards  of  material  from  catch  basins, 
and  14,826  catch-basin  cleanings.  This  made  an  average  of 
two  and  one-half  cleanings  per  catch-basin  per  year,  or  1J  cubic 
yards  removed  at  each  cleaning.  The  6,432  catch-basins  were 
inspected  71,890  times.  There  were  4,112  cubic  yards  of  material 
removed  from  517  miles  of  sewers,  or  about  8  cubic  yards  per 
mile.  Inspection  of  194  miles  of  brick  sewers  were  made,  .4.4 
miles  were  flushed,  and  27  miles  were  cleaned.  Inspections  of  198 
miles  of  pipe  sewers  were  made,  80  miles  were  examined  more 
closely,  37  miles  were  flushed,  and  91  miles  were  cleaned.  The 
field  organization  for  this  work  consisted  of  17  foremen,  8 
assistant  foremen,  29  laborers,  71  cleaners,  13  mechanics,  7 
inspectors  of  construction,  3  inspectors  of  sewer  connections, 
13  horses  and  wagons,  and  28  horses  and  carts.1 
1  Mun.  Journal,  Vol.  36,  1914,  p.  736. 
332 


CAUSES  OF  TROUBLES  333 

199.  Causes  of  Troubles. — The  complaints  most  frequently 
received  about  sewers  are  caused  by  clogging,  breakage  of  pipes, 
and  bad  odors.     Sewers  become  clogged  by  the  deposition  of  sand 
and  other  detritus  which  results  in  the  formation  of  pools  in 
which  organic  matter  deposits,  aggravating  the  clogged  condi- 
tion of  the  sewers  and  causing  the  odors  complained  of.     Grease 
is  a  prolific  cause  of  trouble.     It  is  discharged  into  the  sewer 
in  hot  wastes,   and  becoming  cooled,   deposits  in  thick  layers 
which  may  effectively  block  the  sewer  if  not  removed.     It  can 
be  prevented  from  entering  the  sewers  by  the  installation  of 
grease  traps  as  described  in  Chapter  VI.     The  periodic  cleaning 
of  these  traps  is  as  important  as  their  installation. 

Tree  roots  are  troublesome,  particularly  in  small  pipe  sewers 
in  residential  districts.  Roots  of  the  North  Carolina  poplar, 
silver  leaf  poplar,  willow,  elm,  and  other  trees  will  enter  the 
sewer  through  minute  holes  and  may  fill  the  sewer  barrel  com- 
pletely if  not  cut  away  in  time.  Fungus  growths  occasionally 
cause  trouble  in  sewers  by  forming  a  network  of  tendrils  that 
catches  floating  objects  and  builds  a  barricade  across  the  sewer. 
Difficulties  from  fungus  growths  are  not  common,  but  constant 
attention  must  be  given  to  the  removal  of  grit,  grease,  and  roots. 
Tarry  deposits  from  gas-manufacturing  plants  are  occasionally 
a  cause  of  trouble,  as  they  cement  the  detritus,  already  deposited 
into  a  tough  and  gummy  mass  that  clings  tenaciously  to  the 
sewer. 

Broken  sewers  are  caused  by  excessive  superimposed  loads, 
undermining,  and  progressive  deterioration.  The  changing  char- 
acter of  a  district  may  result  in  a  change  of  street  grade,  an  increase 
in  the  weight  of  traffic,  or  in  the  construction  of  other  structures 
causing  loads  upon  the  sewer  for  which  it  was  not  designed. 
The  presence  of  corrosive  acids  or  gases  may  cause  the  deteriora- 
tion of  the  material  of  the  sewer. 

200.  Inspection. — The    maintenance    of    a    sewerage    system 
is  usually  placed  under  the  direction  of  a  sewer  department.     In 
the   organization   of  the  work  of  this  department   no   regular 
routine  of  inspection  of  all  sewers  need  be  followed  ordinarily. 
Attention  should  be  given  regularly  to  those  sewers  that  are 
known  to   give   trouble,   whereas   the   less   troublesome   sewers 
need  not  be  inspected  more  frequently  than  once  a  year,  pref- 
erably during  the  winter  when  labor  is  easier  to  obtain. 


334 


MAINTENANCE  OF  SEWERS 


The  routine  inspection  of  sewers  too  small  to  enter  is  made  by 
an  examination  at  the  manhole.  If  the  water  is  running  as  freely 
at  one  manhole  as  at  the  next  manhole  above,  it  is  assumed 
that  the  sewer  between  the  manholes  is  clean  and  no  further 
inspection  need  be  given  unless  there  is  some  other  reason  to 
suspect  clogging  between  manholes.  If  the  sewage  is  backed 
up  in  a  manhole  it  indicates  that  there  is  an  obstruction  in  the 
sewer  below.  If  the  sewage  in  a  manhole  is  flowing  sluggishly 
and  is  covered  with  scum  it  is  an  indication  of  clogging,  slow 
velocity  and  septic  action  in  the  sewer.  Sludge  banks  on  the  slop- 
ing bottom  of  the  manhole  or  signs  of  sewage  high  upon  the 


Mirror 


Reflected 
Sunlight* 


Obstruction 


Obi 


Y/7////////////A  ^r  ^  V//////////////////A 

FIG.  140. — Inspecting  Sewers  with  Reflected  Sunlight. 


walls  indicate  an  occasional  flooding  of  the  sewer  due  to  inade- 
quate capacity  or  clogging. 

If  any  of  the  signs  observed  indicate  that  the  sewer  is  clogged, 
the  manhole  should  be  entered  and  the  sewer  more  carefully 
inspected.  Such  inspection  may  be  made  with  the  aid  of  mirrors 
as  shown  in  Fig.  140  or  with  a  periscope  device  as  shown  in  Fig. 
141.  Sunlight  is  more  brilliant  than  the  electric  lamp  shown 
in  Fig.  141,  but  the  mirror  in  the  manhole  directs  the  sunlight 
into  the  eyes  of  the  observer,  dazzling  him  and  preventing  a  good 
view  of  the  sides  of  the  sewer.  The  observers'  eyes  can  be  pro- 
tected against  the  direct  rays  of  the  electric  light,  which  can  be 
projected  against  the  sides  of  the  pipe  by  proper  shades  and 
reflectors.  It  is  possible  with  this  device  to  locate  house  con- 


INSPECTION 


335 


nection,  stoppages,  breaks  of  the  pipe,  and  to  determine  fairly 
accurately  the  condition  of  the  sewer  without  discomfort  to  the 
observers. 

Sewers  that  are  large  enough  to  enter  should  be  inspected  by 
walking  through  them  where  possible.  The  inspection  should  be 
conducted  by  cleaning  off  the  sewer  surface  in  spots  with  a 
small  broom,  and  examining  the  brick  wall  for  loose  bricks,  loose 
cement  or  cement  lost  from  the  joints,  open  joints,  broken  bond, 
eroded  invert,  and  such  other  items  as  may  cause  trouble.  An 
inspection  in  storm  sewers  is  sometimes  of  value  in  detecting  the 
presence  of  forbidden  house  connections. 

/ 


FIG.  141. — Inspecting  Sewers  with  Periscope  and  Electric  Light.    The  G-K 

System. 

Certain  precautions  should  be  taken  before  entering  sewers 
or  manholes.  If  a  distinct  odor  of  gasoline  is  evident  the  sewer 
should  be  ventilated  as  well  as  possible  by  leaving  a  number  of 
manhole  covers  open  along  the  line  until  the  odor  of  gasoline 
has  disappeared.  The  strength  of  gasoline  odor  above  which 
it  is  unsafe  to  enter  a  sewer  is  a  matter  of  experience  possessed 
by  few.  A  slight  odor  of  gasoline  is  evident  in  many  sewers 
and  indicates  no  special  danger.  A  discussion  of  the  amount  of 
gasoline  necessary  to  create  explosive  conditions  is  given  in 
Art.  206.  In  making  observations  of  the  odor  it  should  also  be 
noted  whether  air  is  entering  or  leaving  the  manhole.  The 
presence  of  gasoline  cannot  be  detected  at  a  manhole  into  which 
air  is  entering. 


336  MAINTENANCE  OF  SEWERS 

As  soon  as  it  is  considered  that  the  odors  from  a  sewer  indicate 
the  absence  of  an  explosive  mixture,  a  lighted  lantern  or  other 
open  flame  should  be  lowered  into  the  manhole  to  test  the 
presence  of  oxygen.  Carbon  monoxide  or  other  asphyxiating 
gases  may  accumulate  in  the  sewer,  and  if  present  will  extinguish 
the  flame.  If  the  flame  burns  brilliantly  the  sewer  is  probably 
safe  to  enter,  but  if  conditions  are  unknown  or  uncertain,  the  man 
entering  should  wear  a  life  belt  attached  to  a  rope  and  tended 
by  a  man  at  the  surface.  Asphyxiating  or  explosive  gases  are 
sometimes  run  into  without  warning  due  to  their  lack  of  odor, 
or  the  presence  of  stronger  odors  in  the  sewer.  Breathing  masks 
and  electric  lamps  are  precautions  against  these  dangers,  the 
masks  being  ready  for  use  only  when  actually  needed.  More 
deaths  have  occurred  in  sewers  due  to  asphyxiating  gases  than 
by  explosions,  as  the  average  sewer  explosion  is  of  insufficient 
violence  to  do  great  damage,  although  on  occasion,  extremely 
violent  explosions  have  occurred.  During  inspections  of  sewers 
there  should  always  be  at  least  one  man  at  the  surface  to  call 
help  in  case  of  accident  and  the  inspecting  party  should  consist 
of  at  least  two  men. 

It  must  not  be  felt  that  entering  sewers  is  fraught  with  great 
danger,  as  it  is  perfectly  safe  to  enter  the  average  sewer.  The 
air  is  not  unpleasant  and  no  discomfort  is  felt,  but  conditions 
are  such  that  unexpected  situations  may  arise  for  which  the  man 
in  the  sewer  should  be  prepared.  It  is  therefore  wise  to  take 
certain  precautions.  These  may  indicate  to  the  uninitiated,  a 
greater  danger  than  actually  exists. 

The  inspection  of  sewers  should  include  the  inspection  of  the 
flush-tanks,  control  devices,  grit  chambers,  and  other  appurte- 
nances. A  common  difficulty  found  with  flush-tanks  is  that  the 
tank  is  "  drooling,"  that  is  to  say  the  water  is  trickling 
out  of  the  siphon  as  fast  as  it  is  entering  the  tank,  and  the  inter- 
mittency  of  the  discharge  has  ceased.  If,  when  the  tank  is  first 
inspected  the  water  is  about  at  the  level  of  the  top  of  the  bell  it 
is  probable  that  the  siphon  is  drooling.  A  mark  should  be  made 
at  the  elevation  of  the  water  surface  and  the  tank  inspected  again 
in  the  course  of  an  hour  or  more.  If  the  water  level  is  unchanged 
the  siphon  is  drooling.  This  may  be  caused  by  the  clogging  of 
the  snift  hole  or  by  a  rag  or  other  obstacle  hanging  over  the  siphon 
which  permits  water  to  pass  before  the  air  has  been  exhausted, 


CLEANING  SEWERS  337 

or  a  misplacement  of  the  cap  over  the  siphon,  or  other  difficulty 
which  may  be  recognized  when  the  principle  on  which  the  siphon 
operates  is  understood.  Occasionally  it  is  discovered  that  an 
over  zealous  water  department  has  shut  off  the  service. 

Control  devices,  such  as  leaping  or  overflow  weirs,  automatic 
valves,  etc.,  may  become  clogged  and  cease  to  operate  satis- 
factorily. They  should  be  inspected  frequently,  dependent  upon 
their  importance  and  the  frequency  with  which  they  have  been 
found  to  be  inoperative.  An  inspection  will  reveal  the  obstacle 
which  should  be  removed.  Floats  should  be  examined  for  loss 
of  buoyancy  or  leaks  rendering  them  useless.  Grit  and  screen 
chambers  should  be  examined  for  sludge  deposits. 

Catch-basins  on  storm  sewers  are  a  frequent  cause  of  trouble 
and  need  more  or  less  frequent  cleaning.  Cleanings  are  more 
important  than  inspections  for  catch-basins  for  if  they  are 
operating  properly  they  are  usually  in  need  of  cleaning  after  every 
storm  of  any  magnitude,  and  a  regular  schedule  of  cleaning  should 
be  maintained. 

A  record  should  be  kept  of  all  inspections  made.  It  should 
include  an  account  of  the  inspection,  its  date,  the  conditions 
found,  by  whom  made  and  the  remedies  taken  to  effect  repairs. 

201.  Repairs. — Common  repairs  to  sewerage  systems  consist 
in  replacing  street  inlets  or  catch-basin  covers  broken  by  traffic; 
raising  or  lowering  catch-basin  or  manhole  heads  to  compensate 
for  the  sinking  of  the  manhole  or  the  wear  of  the  pavement; 
replacing  of  broken  pipes,  loosened  bricks  or  mortar  which  has 
dropped  out;    and  other  miscellaneous  repairs  as  the  necessity 
may  arise.     Connections  from  private  drains  are  a  source  of 
trouble  because  either  the  sewer  or  the  drain  has  broken  due 
to  careless  work  or  the  settlement  of  the  foundation   or  the 
backfill. 

202.  Cleaning  Sewers. — Sewers  too  small  to  enter  are  cleaned 
by  thrusting  rods  or  by  dragging  through  them  some  one  of  the 
various  instruments  available.     The  common  sewer  rod  shown 
in  Fig.  142  is  a  hickory  stick,  or  light  metal  rod,  3  or  4  feet  long, 
on  the  end  of  which  is  a  coupling  which  cannot  come  undone  in 
the  sewer.     Sections  of  the  rod  are  joined  in  the  manhole  and 
pushed  down  the  sewer  until  the  obstruction  is  reached  and 
dislodged.     Occasionally    pieces    of    pipe    screwed    together    are 
used  with  success.     The  end  section  may  be  fitted  with  a  special 


338 


MAINTENANCE  OF  SEWERS 


cutting  shoe  for  dislodging  obstructions.  In  extreme  cases  these 
rods  may  be  pushed  400  to  500  feet,  but  are  more  effective  at 
shorter  distances.  Obstructions  may  be  dislodged  by  shoving  a 
fire  hose,  which  is  discharging  water  under  high  pressure  through 
a  small  nozzle,  down  the  sewer  toward  the  obstruction.  The 
water  pressure  stiffens  the  hose,  which,  together  with  the  support 
from  the  sides  of  the  conduit,  make  it  possible  to  push  the  hose 
in  for  effective  work  100  feet  or  more  from  the  manhole.  A 
strip  of  flexible  steel  about  J  inch  thick  and  1J  to  2  inches,  wide 
is  useful  for  "  rodding  "  a  short  length  of  crooked  sewer. 

Sewers  are  seldom  so  clogged  that  no  channel  whatever  remains. 
As  a  sewer  becomes  more  and  more  clogged,  the  passage  becomes 
smaller,  thereby  increasing  the  velocity  of  flow  of  the  sewage 


FIG,  142,— Sewer  Rods 


around  the  obstruction  and  maintaining  a  passageway  by  erosion. 
This  phenomenon  has  been  taken  advantage  of  in  the  cleaning  of 
sewers  by  "  pills."  These  consist  of  a  series  of  light  hollow  balls 
varying  in  size.  One  of  the  smaller  balls  is  put  into  the  sewer 
at  a  manhole.  When  the  ball  strikes  an  obstruction  it  is  caught 
and  jammed  against  the  roof  of  the  sewer.  The  sewage  is  backed 
up  and  seeks  an  outlet  around  the  ball,  thus  clearing  a  channel 
and  washing  the  ball  along  with  it.  The  ball  is  caught  at  the 
next  manhole  below.  A  net  should  be  placed  for  catching  the  ball 
and  a  small  dam  to  prevent  the  dislodged  detritus  from  passing 
down  into  the  next  length  of  pipe.  The  feeding  of  the  balls  into 
the  sewer  is  continued,  using  larger  and  larger  sizes,  until  the  sewer 
is  clean.  This  method  is  particularly  useful  for  the  removal  of 
sludge  deposits,  but  it  is  not  effective  against  roots  and  grease. 


CLEANING  SEWERS 


339 


The  balls  should  be  sufficiently  light  to  float.     Hollow  metiil 
balls  are  better  than  heavier  wooden  ones. 

Plows  and  other  scraping  instruments  are  dragged  through 
pipe  sewers  to  loosen  banks  of  sludge  and  detritus  and  to  cut 
roots  or  dislodge  obstructions.  One  form  of  plow  consists  of 
a  scoop  1  similar  to  a  grocer's  sugar  scoop,  which  is  pushed  or 


FIG.  143. — Cable  and  Windlass  Method  of  Cleaning  Sewers. 

The  cable  is  held  to  the  bottom  of  the  sewer  by  bracing  a  2x4  upright  in  the  sewer,  with  a 
snatch  block  attached.     A  trailer  is  attached  to  the  scoop  to  prevent  loss  of  material. 

dragged  up  a  sewer  against  the  direction  of  flow.  As  fast  as  the 
scoop  is  filled  it  is  drawn  back  and  emptied.  The  method  of 
dragging  this  through  a  sewer  is  indicated  in  Fig.  143.  At 
Atlantic  City  the  crew  operating  the  scoop  comprises  five  men, 
two  are  at  work  in  each  manhole  and  one  on  the  surface  to  warn 
traffic  and  wait  on  the  men  in  the  manholes.  The  outfit  of 


FIG.  144. — Sewer  Cleaning  Device. 

Eng.  News,  Vol.  42,  1899,  p.  328. 

tools  is  contained  in  a  hand-drawn  tool  box  and  includes  sewer 
rods,  metal  scoops  for  all  sizes  of  sewers,  picks,  shovels,  hatchets, 
chisels,  lanterns,  grease  and  root  cutters,  etc.,  and  two  winches 
with  from  400  to  600  feet  of  f-inch  wire  cable. 

Another  form  of  plow  or  drag  consists  of  a  set  of  hooks  or 
teeth  hinged  to  a  central  bar  as  shown  in  Fig.  144.     A  root  cutter 
1  Mun.  Journal,  Vol.  39,  1915,  p.  911. 


340 


MAINTENANCE  OF  SEWERS 


and  grease  scraper  in  the  form  of  a  spiral  spring  with  sharpened 
edges,  and  other  tools  for  cleaning  sewers  are  shown  in  Fig.  145. 
A  turbine  sewer  cleaner  shown  in  Fig.  146  consists  of  a  set  of 
cutting  blades  which  are  revolved  by  a  hydraulic  motor  of  about 


FIG.  145.— Tools  for  Cleaning  Sewers. 

3  horse-power  under  an  operating  pressure  of  about  60  pounds 
per  square  inch.  The  turbine  is  attached  to  a  standard  fire 
hose  and  is  pushed  through  the  sewer  by  utilizing  the  stiffness 
of  the  hose,  or  by  rods  attached  to  a  pushing  jack  as  shown  in 
the  figure.  This  machine  was  invented  and  patented  by  W.  A. 


FIG.  146. — Turbine  Sewer  Machine  Connected  to  Forcing  Jack. 

The  forcing  jack  is  used  when  windlass  and  cable  cannot  be  used. 
Courtesy,  The  Turbine  Sewer  Machine  Co. 

Stevenson  in  1914.  Its  performance  is  excellent.  The  blades 
revorve  at  about  600  R.P.M.,  cutting  roots  and  grease.  The 
revolving  blades  and  the  escaping  water  also  serve  to  loosen 
and  stir  up  the  deposits  and  the  forward  helical  motion  imparted 
to  the  water  is  useful  in  pushing  the  material  ahead  of  the  machine 


FLUSHING  SEWERS  341 

and  in  scrubbing  the  walls  of  the  sewer.  In  Milwaukee  four  men 
with  the  machine  cleaned  319  feet  of  12-inch  sewer  in  16  hours, 
and  in  Kansas  City  7,801  feet  of  sewers  were  cleaned  in  14  days. 

Sewers  large  enough  to  enter  may  be  cleaned  by  hand.  The 
materials  to  be  removed  are  shoveled  into  buckets  which  are 
carried  or  floated  to  manholes,  raised  to  the  surface  and  dumped. 
In  very  large  sewers  temporary  tracks  have  been  laid  and  small 
cars  pushed  to  the  manhole  for  the  removal  of  the  material. 
Hydraulic  sand  ejectors  may  also  be  used  for  the  removal  of 
deposits,  similar  to  the  steam  ejector  pump  shown  in  Fig.  97. 
The  water  enters  the  apparatus  at  high  velocity,  under  a  pressure 
of  about  60  pounds  per  square  inch,  leaps  a  gap  in  the  machine 
from  a  nozzle  to  a  funnel-shaped  guide  leading  to  the  discharge 
pipe.  The  suction  pipe  of  the  machine  leads  to  the  chamber 
in  which  the  leap  is  made.  In  leaping  this  gap  the  water  creates 
a  vacuum  that  is  sufficient  to  remove  the  uncemented  detritus 
large  enough  to  pass  through  the  machine,  and  will  lift  small 
stones  to  a  height  of  10  to  12  feet.  Occasionally  barricades  of 
logs,  tree  branches,  rope,  leaves,  and  other  obstructions  which  have 
piled  up  against  some  inward  projecting  portion  of  the  sewer, 
must  be  removed  by  hand  either  by  cutting  with  an  axe  or  by 
pulling  them  out.  Projections  from  the  sides  of  sewers  are 
objectionable  because  of  their  tendency  to  catch  obstacles  and 
form  barricades. 

Little  authentic  information  on  the  cost  of  cleaning  sewers 
is  available.  A  permanent  sewer  organization  is  maintained 
by  many  cities.  The  division  of  their  time  between  repairs, 
cleaning,  and  other  duties  is  seldom  made  a  matter  of  record. 
From  data  published  in  Public  Works  1  it  is  probable  that  the 
cost  varies  from  $3  to  $15  per  cubic  yard  of  material  removed. 
From  the  information  in  Vol.  II  of  "American  Sewerage  Practice" 
by  Metcalf  and  Eddy  the  combined  cost  of  cleaning  and  flushing 
will  vary  between  $10  and  $40  per  mile;  the  expense  of 
either  flushing  or  cleaning  alone  being  about  one-half  of  this. 

203.  Flushing  Sewers. — Sewers  can  sometimes  be  cleaned  or 
kept  clean  by  flushing.  Flushing  may  be  automatic  and  frequent, 
or  hand  flushing  may  be  resorted  to  at  intervals  to  remove 
accumulated  deposits.  Automatic  flush-tanks,  flushing  man- 
holes, a  fire  hose,  a  connection  to  a  water  main,  a  temporary 
1  Formerly  the  Municipal  Journal. 


342  MAINTENANCE  OF  SEWERS 

fixed  dam,  a  moving  dam,  and  other  methods  are  used  in  flushing 
sewers.  The  design,  operation,  and  results  obtained  from  the 
use  of  automatic  flush-tanks  and  flushing  manholes  are  discussed 
in  Chapter  VI. 

The  method  in  use  for  cleaning  a  sewer  by  thrusting  a  fire 
hose  down  it  can  also  be  used  for  flushing  sewers.  It  is  an 
inexpensive  and  fairly  satisfactory  method.  There  is,  however, 
some  danger  of  displacing  the  sewer  pipe  because  of  the  high 
velocity  of  the  water.  An  easier  and  safer  but  less  effective 
method  is  to  allow  water  to  enter  at  the  manhole  and  flow  down 
the  sewer  by  gravity.  Direct  connections  to  the  water  mains 
are  sometimes  opened  for  the  same  purpose. 

Sewers  are  sometimes  flushed  by  the  construction  of  a  tempo- 
rary dam  across  the  sewer,  causing  the  sewage  to  back  up.  When 
the  sewer  is  half  to  three-quarters  full  the  dam  is  suddenly  removed 
and  the  accumulated  sewage  allowed  to  rush  down  the  sewer,  thus 
flushing  it  out.  The  dam  may  be  made  of  sand  bags,  boards  fitted 
to  the  sewer,  or  a  combination  of  boards  and  bags.  The  expense  of 
equipment  for  flushing  by  this  method  is  less  than  that  by  any 
other  method,  but  the  results  obtained  are  not  always  desirable. 
Below  the  dam  the  results  compare  favorably  with  those  obtained 
by  other  methods,  but  above  the  dam  the  stoppage  of  the  flow 
of  the  sewage  may  cause  depositions  of  greater  quantities  of 
material  than  have  been  flushed  out  below.  A  time  should  be 
chosen  for  the  application  of  this  method  when  the  sewage  is 
comparatively  weak  and  free  from  suspended  matter.  The 
most  convenient  place  for  the  construction  of  a  dam  is  at  a  man- 
hole in  order  that  the  operator  may  be  clear  of  the  rush  of  sewage 
when  the  dam  is  removed. 

Movable  dams  or  scrapers  are  useful  in  cleaning  sewers  of  a 
moderate  size,  but  are  of  little  value  in  small  sewers.  The 
scraper  fits  loosely  against  the  sides  of  the  sewer  and  is  pushed 
forward  by  the  pressure  of  the  sewage  accumulated  behind  it. 
The  iron-shod  sides  of  the  dam  serve  to  scrape  grease  and 
growths  attached  to  the  sewer  and  to  stir  up  sand  and  sludge 
deposited  on  the  bottom.  The  high  velocity  of  the  sewage 
escaping  around  the  sides  of  the  dam  aids  in  cleaning  and 
scrubbing  the  sewer. 

A  natural  watercourse  may  be  diverted  into  the  sewer  if 
topographical  conditions  permit,  or  where  sewers  discharge 


CLEANING  CATCH-BASINS  343 

into  the  sea  below  high  tide  a  gate  may  be  closed  during  the 
flood  and  held  closed  until  the  ebb.  The  rush  of  sewage  on  the 
opening  of  the  gate  serves  to  flush  the  sewers  and  stir  up  the 
sludge  deposited  during  high  tide.  Other  methods  of  flushing 
sewers  may  be  used  dependent  on  the  local  conditions  and  the 
ingenuity  of  the  engineer  or  foreman  in  charge. 

In  some  sewers  it  is  not  necessary  to  remove  the  clogging 
material  from  the  sewer.  It  is  sufficient  to  flush  and  push  it 
along  until  it  is  picked  up  and  carried  away  by  higher  velocities 
caused  by  steeper  grades  or  larger  amounts  of  sewage. 

204.  Cleaning  Catch-basins.1 — Catch-basins  have  no  reason 
for  existence  if  they  are  not  kept  clean.  Their  purpose  is  to 
catch  undesirable  settling  solids  and  to  prevent  them  from 
entering  the  sewers,  on  the  theory  that  it  is  cheaper  to  clean  a 
catch-basin  than  it  is  to  clean  a  sewer.  If  the  cleaning  of  storm 
sewers  below  some  inlet  to  which  no  catch-basin  is  attached 
becomes  burdensome,  the  engineer  in  charge  of  maintenance 
should  install  an  adequate  catch-basin  and  keep  it  clean.  Catch- 
basins  are  cleaned  by  hand,  suction  pumps,  and  grab  buckets. 
In  cleaning  by  hand  the  accumulated  water  and  sludge  are 
removed  by  a  bucket  or  dipper  and  dumped  into  a  wagon  from 
which  the  surplus  settled  water  is  allowed  to  run  back  into  the 
sewer.  The  grit  at  the  bottom  of  the  catch-basin  is  removed 
by  shoveling  it  into  buckets  which  are  then  hoisted  to  the 
surface  and  emptied. 

Suction  pumps  in  use  for  cleaning  catch-basins  are  of  the 
hydraulic  eductor  type.  The  eductor  works  on  the  principle  of 
the  steam  pump  shown  in  Fig.  97,  except  that  water  is  used 
instead  of  steam.  The  material  removed  may  be  discharged 
into  settling  basins  constructed  in  the  street,  or  may  be  dis- 
charged directly  into  wagons.2  In  Chicago  a  special  motor- 
driven  apparatus  is  used.  This  consists  of  a  5-yard  body  on  a 
5-ton  truck,  and  a  centrifugal  pump  driven  by  the  truck  motor. 
In  use,  the  truck,  about  half  filled  with  water,  drives  up  to  the 
catch-basin,  the  eductor  pipe  is  lowered  and  water  pumped  from 
the  truck  into  the  eductor  and  back  into  the  truck  again,  together 
with  the  contents  of  the  catch-basin.  The  surplus  water  drains 

1  See  Eng.  Record,  Vol.  75,  1917,  p.  463. 

2Eng.  Record,  Vol.  73,  1916,  p.  141,  and  Eng.  News-Record,  Vol.  79, 
1917,  p.  1019. 


344  MAINTENANCE  OF  SEWERS 

back  into  the  sewer.  The  Chicago  Bureau  of  Sewers  reports 
a  truck  so  equipped  to  have  cleaned  1013  catch-basins,  removing 
1763  cubic  yards  of  material,  and  running  1380  miles,  during  the 
months  of  August,  September  and  October,  1917.  The  cost, 
including  all  items  of  depreciation,  wages,  repairs,  etc.,  was 
$1,393.89.  Orange-peel  buckets,  about  20  inches  in  diameter, 
operated  by  hand  or  by  the  motor  of  a  3J  to  5-ton  truck  with  a 
water-tight  body,  are  used  for  cleaning  catch-basins  in  some  cities. 

Catch-basins  in  unpaved  streets  and  on  steep  sandy  slopes 
should  be  cleaned  after  every  storm  of  consequence.  Basins 
which  serve  to  catch  only  the  grit  from  pavement  washings 
require  cleaning  about  two  or  three  times  per  year,  and  from  one  to 
three  cubic  yards  of  material  are  removed  at  each  cleaning.  The 
cost  of  cleaning  ordinary  catch-basins  by  hand  may  vary  from  $15 
to  $25,  but  with  the  use  of  eductors  or  orange-peel  buckets  the 
cost  is  somewhat  lower.  In  Seattle  the  cost  of  cleaning  large 
detritus  basins  by  hand  is  said  1  to  vary  from  $45  to  $60.  With 
the  use  of  eductors  this  cost  has  been  reduced  to  one-third  or 
one-fifth  the  cost  of  cleaning  by  hand. 

205.  Protection  of  Sewers.2 — City  ordinances  should  be 
wisely  drawn  and  strictly  enforced  for  the  protection  of  sew- 
ers against  abuse  and  destruction.  The  requirements  of  some 
city  ordinances  are  given  in  the  following  paragraphs. 

Washington,  D.  C.,3  sewer  ordinances  provide  that: 

No  person  shall  make  or  maintain  any  connection  with 
any  public  sewer  or  appurtenance  thereof  whereby  there 
may  be  conveyed  into  the  same  any  hot,  suffocating, 
corrosive,  inflammable  or  explosive  liquid,  gas,  vapor, 
substance  or  material  of  any  kind  .  .  .  provided  that 
the  provisions  of  this  act  shall  not  apply  to  water  from 
ordinary  hot  water  boilers  or  residences. 

The  following  extracts  from  the  ordinances  of  Indianapolis 
are  typical  of  those  from  many  cities: 

2950.  No  connection  shall  be  made  with  any  public 
sewer  without  the  written  permission  of  the  Committee 
on  Sewers  and  the  Sewerage  Engineer. 

1  Eng.  Record,  Vol.  72,  1915,  p.  690. 
2Eng.  Record,  Vol.  71,  1915,  p.  256. 
s  Eng.  and  Contr.,  Vol.  41,  1914,  p.  250. 


PROTECTION  OF  SEWERS  345 

2953.  No  person  shall  be  authorized  to  do  the  work 
of  making  connections  until  he  has  furnished  a  satisfactory 
certificate  that  he  is  qualified  for  the  duties.  He  shall 
also  file  bond  for  not  less  than  $1,000  that  he  will  indemnify 
the  City  from  all  loss  or  damage  that  may  result  from  his 
work  and  that  he  will  do  the  work  in  conformity  to  the 
rules  and  regulations  established  by  the  City  Council. 

2955.  It  shall  be  unlawful  for  any  person  to  allow 
premises   connected  to   the  sewers  or   drains  to   remain 
without  good  fixtures  so  attached  as  to  allow  a  sufficiency 
of  water  to  be  applied  to  keep  the  same  unobstructed. 

2956.  No  butcher's  offal  or  garbage,  or  dead  animals, 
or  obstructions  of  any  kind  shall  be  thrown  in  any  receiving 
basin  or  sewer  in  penalty  not  greater  than  $100.     Any 
person    injuring,   breaking,   or  removing   any   portion   of 
any  receiving  basin,  manhole  cover,  etc.,  shall  be  fined 
not  more  than  $100. 

2962.  No  person  shall  drain  the  contents  of  any  cess- 
pool or  privy  vault  into  any  sewer  without  the  permission 
of  the  Common  Council. 

The    Cleveland    ordinances    are    similar    and    contain    the 
following  in  addition: 

1251.  Rule  4.  All  connections  with  the  main  or  branch 
sewers  shall  be  made  at  the  regular  connections  or  junctions 
built  into  the  same,  except  by  special  permit. 

Rule  16.  No  stean  pipe,  nor  the  exhaust,  nor  the  blow 
off  from  any  steam  engine  shall  be  connected  with  any 
sewer. 

Evanston,  Illinois,  protects  its  sewers  against  the  additions  of 
grease  and  other  undesirable  substances  as  follows: 

1444.  It  is  unlawful  for  any  person  to  use  any  sewer 
or  appurtenance  to  the  sewerage  system  in  any  manner 
contrary  to  the  orders  of  the  Commissioner  of  Public  Works. 

1446.  Wastes  from  any  kitchen  sinks,  floor  drains,  or 
other  fixtures  likely  to  contain  greasy  matter  from  hotels, 
certain  apartment  houses,  boarding  houses,  restaurants, 
butcher  shops,  packing  houses,  lard  rendering  establish- 
ments, bakeries,  launderies,  cleaning  establishments,  gar- 
ages, stables,  yard  and  floor  drains,  and  drains  from 
gravel  roofs  shall  be  made  through  intervening  receiving 
basins  constructed  as  prescribed  in  par.  VIII  of  this  code. 

Receiving  basins  suitable  for  the  work  required  in  the  code 
are  illustrated  in  Chapter  VI. 


346  MAINTENANCE  OF  SEWERS 

206.  Explosions  in  Sewers. — Disastrous  explosions  in  sewers 
were  first  recorded  about  1886.1  Up  to  about  1905  explosions 
were  infrequent  and  were  considered  as  unavoidable  accidents 
and  so  rare  as  to  be  unworthy  of  study.  For  a  decade  or  more 
after  1905  explosions  occurred  with  increasing  violence  and 
frequency  causing  destruction  of  property,  but  by  some  freakish 
chance,  but  little  loss  of  life.  A  violent  and  destructive  explosion 
occurred  in  Pittsburgh  on  Nov.  25,  19 13,2  and  another  on  March 
12,  1916.  The  property  damage  amounted  to  $300,000  to 
$500,000  on  each  occasion,  but  there  was  no  loss  of  life.  Two 
miles  of  pavement  were  ripped  up,  gas,  water,  and  other  sewer 
pipes  were  broken,  buildings  collapsed  and  the  streets  were 
flooded.  The  streets  were  rendered  unserviceable  for  long 
periods  during  the  expensive  repairs  that  were  necessary.  In 
recent  years  the  number  of  explosions  in  sewers  has  been  smaller, 
due  probably  to  the  gain  in  knowledge  of  the  causes  and  intelli- 
gent methods  of  prevention. 

The  three  principal  causes  of  explosions  in  sewers  are :  gasoline 
vapor,  illuminating  gas,  and  calcium  carbide.  It  is  probable 
that  gasoline  vapor  is  by  far  the  most  troublesome.  Explo- 
sions caused  by  these  gases  are  not  so  violent  as  those  caused 
by  dynamite  or  other  high  explosives,  as  the  volume  of  gas  and 
the  temperature  generated  are  much  less.  The  violence  of  sewer 
explosions  may  be  increased  somewhat  by  the  sudden  pressures 
that  are  put  upon  them. 

Gasoline  finds  its  way  into  sewers  from  garages  and  cleaning 
establishments.  A  mixture  of  1J  per  cent  gasoline  vapor  and 
air  may  be  explosive.  It  needs  only  the  stray  spark  of  an 
electric  current,  a  lighted  match,  or  a  cigar  thrown  into  the  sewer 
to  cause  the  explosion.  As  the  result  of  a  series  of  experiments 
on  2,706  feet  of  8-foot  sewer,  Burrell  and  Boyd  conclude:3 

One  gallon  of  gasoline  if  entirely  vaporized  produces 
about  32  cubic  feet  of  vapor  at  ordinary  temperature 
and  pressure.  If  1J  per  cent  be  adopted  as  the  low  explo- 
sive limit  of  mixtures  of  gasoline  vapor  and  air,  55  gallons 
or  a  barrel  of  gasoline  would  produce  enough  vapor  to 
render  explosive  the  mixture  in  1,900  feet  of  9  foot  sewer 

1 H.  J.  Kellogg  in  Journal  Connecticut  Society  of  Civil  Engineers,  1914, 
and  Technical  Paper  117,  U.  S.  Bureau  of  Mines. 
2Eng.  News,  Vol.  70,  1913,  p.  1157. 
« Technical  Paper  No.  117,  U.  S.  Bureau  of  Mines. 


EXPLOSIONS  IN  SEWERS  347 

provided  the  gasoline  and  the  air  were  perfectly  mixed. 
Many  different  factors,  however,  govern  explosibility, 
such  as:  size  of  the  sewer,  velocity  of  the  sewage,  tem- 
perature of  the  sewer,  volatility  and  rate  of  inflow  of  the 
gasoline.  Only  under  identical  conditions  of  tests  would 
duplicate  results  be  obtained.  A  large  amount  of  gaso- 
line poured  in  at  one  time  is  less  dangerous  than  the  same 
amount  allowed  to  run  in  slowly.  With  a  velocity  of 
flow  of  about  6|  feet  per  second  it  was  evident  that  55 
gallons  of  gasoline  poured  all  at  once  into  a  manhole 
rendered  the  air  explosive  only  a  few  minutes  (less  than 
10)  at  any  particular  point.  With  the  same  amount  of 
gasoline  run  in  at  the  rate  of  5  gallons  per  minute,  an 
explosive  flame  would  have  swept  along  the  sewer  if  ignited 
15  minutes  after  the  gasoline  had  been  dumped.  With 
a  slow  velocity  of  flow  and  a  submerged  outlet  the  gasoline 
vapor  being  heavier  than  air  accumulated  at  one  point 
and  extremely  explosive  conditions  could  result  from  a 
small  amount  of  gasoline.  Comparatively  rich  explosive 
mixtures  were  found  5  hours  after  the  gasoline  had  been 
discharged.  High-test  gasoline  is  much  more  dangerous 
than  the  naphtha  used  in  cleaning  establishments,  yet 
on  account  of  the  large  quantity  of  waste  naphtha  the 
sewage  from  cleaning  establishments  may  be  very  dan- 
gerous. 

Illuminating  gas  is  not  so  dangerous  as  gasoline  vapor  as  it  is 
lighter  than  air  and  it  is  more  likely  to  escape  from  the  sewer 
than  to  accumulate  in  it.  It  requires  about  one  part  of 
illuminating  gas  to  seven  parts  of  air  to  produce  an  explosive 
mixture. 

Calcium  carbide  is  dangerous  because  it  is  self  igniting.  The 
heat  of  the  generation  of  gas  is  sufficient  to  ignite  the  explosive 
mixture.  The  gases  are  highly  explosive  and  cause  a  relatively 
powerful  explosion.  Fortunately  large  amounts  of  this  material 
seldom  reach  a  sewer,  the  gas  being  generated  in  garage  drains 
or  traps  and  escaping  in  the  atmosphere. 

A  .hydrocarbon  oil  used  by  railroads  in  preventing  the  freezing 
of  switches,  if  allowed  to  reach  the  sewers,  may  cause  explosions 
therein.1  The  oil  crystallizes  and  in  this  form  it  is  soluble  in 
water.  It  will  thus  pass  traps  and  on  volatilization  will  pro- 
duce explosive  mixtures. 

Methane,  generated  by  the  decomposition  of  organic  matter, 

1  Eng.  News,  Vol.  71,  1914,  p.  84. 


348  MAINTENANCE  OF  SEWERS 

is   a   feebly   explosive   gas   occasionally   found   in   sewers.     Its 
presence  may  add  to  the  strength  of  other  explosive  mixtures. 

Sewer  explosions  may  be  prevented  by  the  building  of  proper 
forms  of  intercepting  basins  to  prevent  the  entrance  of  gasoline 
and  calcium  carbide  gases,  and  by  ventilation  to  dilute  the 
explosive  mixtures  which  may  be  made  up  in  the  sewer.  There 
are  no  practical  means  to  predict  when  an  explosion  is  about  to 
occur,  and  after  an  explosion  has  occurred  it  is  difficult  to  deter- 
mine the  cause  as  all  evidence  is  usually  destroyed. 

207.  Valuation  of  Sewers. — The  necessity  for  the  valuation 
of  a  sewerage  system  may  arise  from  the  legal  provisions  in  some 
states  limiting  the  amount  of  outstanding  bonds  which  may  be 
issued  by  a  municipality  to  a  certain  percentage  of  the  present 
worth  of  municipal  property.  The  investment  in  the  sewerage 
system  is  usually  great  and  forms  a  large  portion  of  the  City's 
tangible  property.  It  may  be  desirable  also  to  determine  the 
depreciation  of  the  sewers  with  a  view  towards  their  renewal. 

The  most  valuable  work  on  the  valuation  of  sewers  has  been 
done  in  New  York  City  1  by  the  engineers  of  the  Sewer  Depart- 
ment. The  committee  of  engineers  appointed  to  do  the  work 
recommended:  (1)  that  the  original  cost  be  made  the  basis  of 
valuation,  and  that  (2),  in  fixing  this  cost  the  cost  of  pavement 
should  be  omitted  or  at  most  the  cost  of  a  cheap  (cobblestone) 
pavement  should  be  included.  Trenches  previously  excavated 
in  rock  were  considered  as  undepreciated  assets. 

The  present  worth  of  sewers  depends  on  many  factors  aside 
from  the  effects  of  age,  such  as  the  care  exercised  in  the  original 
construction,  the  material  used,  the  kind  and  quantity  of  sewage 
carried,  the  care  taken  in  maintenance,  and  finally  the  injury 
caused  by  the  careless  building  of  adjoining  substructures.  Dur- 
ing the  progress  of  the  inspections  the  examination  of  brick  sewers, 
due  to  their  accessibility,  yielded  better  results  than  the  examina- 
tion of  pipe  sewers.  The  routine  of  the  examination  of  the 
brick  sewers  consisted  in  cleaning  off  the  bricks  with  a  short 
broom,  tapping  the  brick  with  a  light  hammer  to  determine 
solidity,  and  testing  the  cement  joints  by  scraping  with  a  chisel. 
In  addition,  measurements  of  height  and  width  were  taken  every 
30  feet.  The  bricks  in  the  invert  at  and  below  the  flow  line 
were  examined  for  wear. 

1  Eng.  News,  Vol.  71,  1914,  p.  82.' 


VALUATION  OF  SEWERS 


349 


A  study  of  the  reports  of  these  examinations  disclosed  that  the 
following  defects  were  noticeable: 

1.  Cement  partly  out  at  water  line. 

2.  Cement  partly  out  above  water  line. 

3.  Depressed  arch  and  sewer  slightly  spread. 

4.  Large  open  joints. 

5.  Loose  brick. 

6.  Bond  of  brick  broken. 

7.  Distorted  sides,  uneven  bottom,  joints  out  of  line. 


in  Years. 


100 


Date  of  Construction 
a.  b. 

FIG.  147. — Diagrams  used  in  Estimating  Depreciation  of  Brick  Sewers  Due  to 
Age,  Manhattan  Borough,  New  York  City. 

o.  Proportionate  deterioration  from  various  causes. 

b.  Percentage  of  depreciation  based  on  examination  of  sewers,  use  of  deterioration  curve 
(Fig.  a),  and  age  of  sewers  examined. 

Eng.  News,  Vol.  71,  p.  84. 

Inspection  of  pipe  sewers  from  manholes,  the  pipe  being  illu- 
minated by  floating  candles,  was  found  to  be  unsatisfactory. 
Reliance  was  placed  on  the  reports  of  men  experienced  in  making 
connections  and  repairs  to  the  sewers.  Early  pipe  sewers  in  New 
York  were  laid  directly  on  the  bottom  of  the  trench.  Under  these 
circumstances  a  small  leak  at  a  joint  was  sufficient  to  wash  the 
earth  away  and  to  drop  the  pipe,  causing  serious  conditions 
along  the  line.  No  wear  or  deterioration  of  pipe  sewers  were 
noted,  the  only  defects  being  cracking  of  the  pipes  at  the  center 
line  due  to  poor  foundation  and  to  defects  in  the  pipe  itself. 


350 


MAINTENANCE  OF  SEWERS 


The  depreciation  of  brick  sewers  as  studied  in  New  York, 
is  shown  graphically  in  Fig.  147.  At  zero  the  sewer  is  in  good 
condition  and  at  100  it  is  in  such  a  state  of  dilapidation  as  to 
require  instant  rebuilding.  Repairs  are  not  considered  economical 
in  this  condition.  In  the  preparation  of  this  diagram  each 
condition  on  the  list  above  was  given  a  certain  number  of  points, 


Age  in  Years. 


—     <T>f-in«o   —    <y>f-iOK> 

Ift     •*-'**-*««-K>»OIOK> 


32 
-S36 

n: 


o  52 

&  56 

-£  60 

S  64 

I  68 

72 

76 

80 

84 

88 

92 

96 

100 


•<&* 


^ 


K 


CJ  «t  \O  tO  O  CU 
«9  IB  u>  vo  f-  r- 
CO  CO  00  00  00 


S8 


Date  of  Construction 


FIG.  148. — Diagram  Showing  Rate  of  Depreciation  of  Pipe  Sewers. 

Eng.  News,  Vol.  71,  p.  86. 


which  when  added  together  represented  the  state  of  depreciation 
of  the  sewer.  These  sums  were  plotted  as  ordinates  and  the 
corresponding  ages  of  the  sewer  were  plotted  as  abscissas.  The 
various  points  were  taken  cumulatively,  and  where  the  bond 
of  the  brickwork  was  broken  (given  a  value  of  72)  plus  other 
defects  gave  a  total  of  164  the  sewer  was  considered  as  valueless 
and  not  worth  repair.  The  scale  of  164  was  later  reduced  to 
a  percentage  basis  as  shown  on  the  right  of  the  figure.  Fig. 
148  shows  a  similar  diagram  for  the  depreciation  of  pipe  sewers. 


VALUATION  OF  SEWERS  351 

It  was  concluded  that  the  life  of  a  brick  sewer  in  New  York 
is  64  years.  Some  of  the  sewers  examined  were  over  200  years 
old.  The  total  original  cost  of  483  miles  of  brick,  pipe  and  wood 
sewers  was  figured  as  $23,880,000  with  a  present  worth  of 
$18,665,000  and  an  average  annual  depreciation  of  2.2  per  cent. 
In  figuring  these  amounts  no  account  was  taken  of  obsolescence. 
The  deterioration  of  catch-basins  proceeded  at  about  the  same 
rate  as  for  brick  sewers. 


CHAPTER  XIII 
COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

208.  Physical  Characteristics. — Sewage  is  the  spent  water 
supply  of  a  community  containing  the  wastes  from  domestic, 
industrial,  or  commercial  use,  and  such  surface  and  ground  water 
as  may  enter  the  sewer.1  Sewages  are  classed  as:  domestic 
sewage,  industrial  waste,  storm  water,  surface  water,  street 
wash,  and  ground  water.  Domestic  sewage  is  the  liquid  dis- 
charged from  residences  or  institutions  and  contains  water 
closet,  laundry,  and  kitchen  wastes.  It  is  sometimes  called  sani- 
tary sewage.  Industrial  sewage  is  the  liquid  waste  resulting 
from  processes  employed  in  industrial  establishments.  Storm 
water  is  that  part  of  the  rainfall  which  runs  over  the  surface  of 
the  ground  during  a  storm  and  for  such  a  short  period  following 
a  storm  as  the  flow  exceeds  the  normal  and  ordinary  run-off. 
Surface  water  is  that  part  of  the  rainfall  which  runs  over  the 
surface  of  the  ground  some  time  after  a  storm.  Street  wash 
is  the  liquid  flowing  on  or  from  the  street  surface.  Ground 
water  is  water  standing  in  or  flowing  through  the  ground  below 
its  surface. 

Ordinary  fresh  sewage  is  gray  in  color,  somewhat  of  the  appear- 
ance of  soapy  dish  water.  It  contains  particles  of  suspended 
matter  which  are  visible  to  the  naked  eye.  If  the  sewage  is 
fresh  the  character  of  some  of  the  suspended  matter  can  be  dis- 
tinguished as:  matches,  bits  of  paper,  fecal  matter,  rags,  etc. 
The  amount  of  suspended  matter  in  sewage  is  small,  so  small  as 
to  have  no  practical  effect  on  the  specific  gravity  of  the  liquid 
nor  to  necessitate  the  modification  of  hydraulic  formulas 
developed  for  application  to  the  flow  of  water.  The  total 
suspended  matter  in  a  normal  strong  domestic  sewage  is  about 
500  parts  per  1,000,000.  It  is  represented  graphically  in  Fig. 
149.  The  quantity  of  organic  or  volatile  suspended  matter 
1  Similar  to  definition  proposed  by  the  Am.  Public  Health  Ass'n. 

352 


PHYSICAL  CHARACTERISTICS 


353 


is  about  200  parts  per  1,000,000.     It  is  shown  graphically  in  the 
smaller  cube  in  Fig.  149. 

The  odor  of  fresh  sewage  is  faint  and  not  necessarily  unpleasant. 
It  has  a  slightly  pungent  odor,  somewhat  like  a  damp  unven- 
tilated  cellar.  Occasionally  the  odor  of  gasoline,  or  some  other 
predominating  waste  matter  may  hide  all  other  odors.  Stale 
sewage  is  black  and  gives  off 
nauseating  odors  of  hydrogen 
sulphide  and  other  gases.  If 
the  sewage  is  so  stale  as  to 
become  septic,  bubbles  of  gas 
will  be  seen  breaking  the  sur- 
face and  a  black  or  gray  scum 
may  be  present.  Before  the 
South  Branch  of  the  Chicago 
River  was  cleaned  up  and 
flushed  this  scum  became  so 
thick  in  places,  particularly  in 
that  portion  of  the  Stock  Yards 
where  the  river  became  known 
as  Bubbly  Creek,  that  it  is  said 
that  weeds  and  small  bushes 
"sprouted  in  it,  and  chickens 
and  small  animals  ran  across 
its  surface. 

A  physical  analysis  of  sewage  should  include  an  observation 
of  its  appearance,  and  a  determination  of  its  temperature,  tur- 
bidity, color,  and  odor,  both  hot  and  cold.  The  temperature  is 
useful  in  indicating  certain  of  the  antecedents  of  the  sewage, 
its  effect  on  certain  forms  of  bacterial  life,  and  its  effect  on  the 
possible  content  of  dissolved  gases.  Temperatures  higher  than 
normal  are  indicative  of  the  presence  of  trades  wastes  discharged 
while  hot  into  the  sewers.  A  low  temperature  may  indicate 
the  presence  of  ground  water.  If  the  temperature  is  much  over 
40°  C.  bacterial  action  will  be  inhibited  and  the  content  of  dis- 
solved gases  will  be  reduced.  Turbidity,  color,  and  odor  deter- 
minations may  be  of  value  in  the  control  of  treatment  devices, 
or  to  indicate  the  presence  of  certain  trades  wastes,  which  give 
typical  reactions.  Since  all  normal  sewages  are  high  in  color 
and  turbidity,  the  relative  amounts  of  these  two  constituents 


FIG.  149. — Graphical  Representation 
of  Relative  Volumes  of  Liquids  and 
Solids  in  Sewage. 


354         COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

in  two  different  sewages  has  little  significance  regarding  the 
relative  strengths  of  the  two  .sewages  or  the  proper  method  of 
treating  them.  A  fresh  domestic  sewage  should  have  no  highly 
offensive  odor.  The  presence  of  certain  trades  wastes  can  be 
detected  sometimes  in  fresh  sewages,  and  a  stale  sewage  may 
sometimes  be  recognized  by  its  odor. 

Sewage  is  a  liability  to  the  community  producing  it.  Although 
some  substances  of  value  can  be  obtained  from  sewage  l  the  cost 
of  the  processes  usually  exceed  the  value  of  the  substances 
obtained.  Where  it  becomes  necessary  to  treat  sewage  the  value 
of  these  substances  may  be  helpful  in  defraying  the  cost  of 
treatment. 

209.  Chemical  Composition. — Sewage  is  composed  of  mineral 
and  organic  compounds  which  are  either  in  solution  or  are  sus- 
pended in  water.  In  making  a  standard  chemical  analysis  of 
sewage  only  those  chemical  radicals  and  elements  are  determined 
which  are  indicative  of  certain  important  constituents.  Neither 
a  complete  qualitative  nor  quantitative  analysis  is  made.  A 
sewage  analysis  will  not  show,  therefore,  the  number  of  grams  of 
sodium  chloride  present  or  any  other  constituent.  A  complete 
standard  sanitary  chemical  analysis  will  report  the  constituents 
as  named  in  the  first  column  of  Table  71.  The  quantities  of 
these  materials  found  in  average  strong,  medium  and  weak 
sewages  are  also  shown  in  this  table.  These  values  are  not 
intended  as  fixed  boundaries  between  sewages  of  different 
strengths.  They  are  presented  merely  as  a  guide  to  the  inter- 
pretation of  sewage  analyses. 

The  principal  objects  of  a  chemical  analysis  of  sewage  are  to 
determine  its  strength  and  its  state  of  decomposition.  The 
influents  and  effluents  of  a  sewage  treatment  device  are  analyzed 
to  aid  in  the  control  of  the  device  and  to  gain  information  con- 
cerning the  effect  of  the  treatment.  Chemical  and  other  analyses, 
in  connection  with  the  desired  conditions  after  disposal,  will 
indicate  the  extent  of  treatment  which  may  be  required.  The 
standard  methods  of  water  and  sewage  analysis  adopted  by  the 
American  Public  Health  Association  have  been  generally  accepted 
by  sanitarians.  These  uniform  methods  make  possible  com- 

1  Economic  Values  in  Sewage  and  Sewage  Sludge,  by  Raymond  Wells, 
Proceedings  Am.  Society  Municipal  Improvements,  Nov.  12,  1919.  Eng. 
News-Record,  Vol.  83,  1919,  p.  948. 


CHEMICAL  COMPOSITION 


355 


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356          COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

parisons  of  the  results  obtained  by  laboratories  working  according 
to  these  standards. 

210.  Significance  of  Chemical  Constituents. — Organic  nitro- 
gen and  free  ammonia  taken  together  are  an  index  of  the  organic 
matter  in  the  sewage.  Organic  nitrogen  includes  all  of  the 
nitrogen  present  with  the  exception  of  that  in  the  form  of  ammonia, 
nitrites,  and  nitrates.  Free  ammonia  or  ammonia  nitrogen  is 
the  result  of  bacterial  decomposition  of  organic  matter.  A  fresh 
cold  sewage  should  be  relatively  high  in  organic  nitrogen  and 
low  in  free  ammonia.  A  stale  warm  sewage  should  be  relatively 
high  in  free  ammonia  and  low  in  organic  nitrogen.  The  sum  of 
the  two  should  be  unchanged  in  the  same  sewage. 

Nitrites  (RN02)  and  nitrates  (RNOs) 1  are  found  in  fresh 
sewages  only  in  concentrations  of  less  than  one  part  per  million. 
In  well-oxidized  effluents  from  treatment  plants  the  concen- 
tration will  probably  be  much  higher.  Nitrates  contain  one  more 
atom  of  oxygen  than  nitrites.  They  represent  the  most  stable 
form  of  nitrogenous  matter  in  sewage.  Nitrites  are  not  stable 
and  are  reduced  to  ammonias  or  are  oxidized  to  nitrates.  Their 
presence  indicates  a  process  of  change.  They  are  not  found  in 
large  quantities  in  raw  sewage  because  their  formation  requires 
oxygen  which  must  be  absorbed  from  some  other  source  than 
the  sewage.  In  an  ordinary  sewer  or  sluggishly  flowing  open 
stream  this  absorption  cannot  take  place  from  the  atmosphere 
with  sufficient  rapidity  to  supply  the  necessary  oxygen. 

Oxygen  consumed  is  an  index  of  the  amount  of  carbonaceous 
matter  readily  oxidizable  by  potassium  permanganate.  It  does 
not  indicate  the  total  quantity  of  any  particular  constituent, 
but  it  is  the  most  useful  index  of  carbonaceous  matter.  Car- 
bonaceous matter  is  usually  difficult  of  treatment  and  a  high 
oxygen  consumed  is  indicative  of  a  sewage  difficult  to  care  for. 
The  amount  of  oxygen  consumed,  as  expressed  in  the  analysis, 
is  dependent  on  the  amount  of  oxidizable  carbonaceous  matter 
present,  the  oxidizing  agent  used,  and  the  time  and  temperature 
of  contact  of  the  sewage  and  the  oxidizing  agent.  It  is  essential 
therefore  that  the  test  be  conducted  according  to  some  standard 
method,  since  the  results  are  of  value  only  as  compared  with 
results  obtained  under  similar  conditions. 

Total  solids  (residue  on  evaporation)  are  an  index  of  the 
1 R  represents  any  chemical  element  such  as  K,  Na,  etc. 


SIGNIFICANCE  OF  CHEMICAL  CONSTITUENTS          357 

strength  of  the  sewage.  They  are  made  up  of  organic  and 
inorganic  substances.  The  inorganic  substances  include  sand, 
clay,  and  oxides  of  iron  and  aluminum,  which  are  usually  insolu- 
ble, and  chlorides,  carbonates,  sulphates  and  phosphates,  which 
are  usually  soluble.  The  insoluble  inorganic  substances  are 
undesirable  in  sewage  because  of  their  sediment  forming  prop- 
erties which  result  in  the  clogging  of  sewers,  treatment  plants, 
pumps,  and  stream  beds.  The  soluble  inorganic  substances  are 
generally  harmless  and  cause  no  nuisance,  except  that  the 
presence  of  sulphur  may  permit  the  formation  of  hydrogen 
sulphide,  which  has  a  highly  offensive  odor.  The  organic  sub- 
stances are:  carbo-hydrates,  fats,  and  soaps,  which  are  car- 
bonaceous and  are  difficult  of  removal  by  biological  processes; 
and  the  nitrogenous  substances  such  as  urea,  proteins,  amines, 
and  amino  acids.  The  inorganic  and  organic  substances  may  be 
either  in  solution  or  suspension  or  in  a  colloidal  condition. 

Volatile  solids  are  used  as  an  index  of  the  organic  matter 
present,  as  it  is  assumed  that  the  organic  matter  is  more  easily 
volatilized  than  the  inorganic  matter.  The  amount  of  volatile 
inorganic  matter  present  is  usually  so  small  as  to  be  negligible. 

Fixed  solids  are  reported  as  the  difference  between  the  total 
and  volatile  solids.  They  are  therefore  representative  of  the 
amount  of  inorganic  matter  present. 

Suspended  matter  is  the  undissolved  portion  of  the  total 
solids.  High  volatile  suspended  matter  is  an  indication  of 
offensive  qualities  in  the  nature  of  putrefying  organic  matter, 
whereas  fixed  suspended  matter  is  indicative  of  inoffensive 
inorganic  matter.  It  is  difficult  to  obtain  a  sample  of  sewage 
which  will  represent  the  amount  of  suspended  matter  in  the 
sewage,  since  a  sample  taken  from  near  the  surface  will  contain 
less  inorganic  matter  and  grit  than  a  sample  taken  near  the  bottom. 

Settling  solids  are  indicative  of  the  sludge  forming  properties 
of  the  sewage  and  of  the  probable  degree  of  success  of  treatment 
by  plain  sedimentation.  Volatile  settling  solids  indicate  the 
property  of  the  formation  of  offensive  putrefying  sludge  banks. 
There  is  no  chemical  test  which  will  indicate  the  scum-forming 
properties  of  sewage.  Fixed  settling  solids  indicate  the  presence 
of  inorganic  matter,  probably  gritty  material  such  as  sand,  clay, 
iron  oxide,  etc. 

Colloidal  matter  is  material  which  is  too  finely  divided  to  be 


358          COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

removed  by  filtration  or  sedimentation,  yet  is  not  held  in  solu- 
tion. It  can  sometimes  be  removed  by  violent  agitation  in  the 
presence  of  a  flocculent  precipitate,  as  in  the  treatment  with 
activated  sludge,  or  by  the  flocculent  precipitate  alone,  as  in 
chemical  precipitation,  or  by  the  acidulation  of  the  sewage  so 
as  to  precipitate  the  colloids.  Colloidal  matter  is  probably  the 
result  of  the  constant  abrasion  of  finely  divided  suspended  matter 
while  flowing  through  the  sewer  or  other  channel.  High  colloidal 
matter  may  therefore  indicate  a  stale  sewage,  or  the  presence  of  a 
particular  trades  waste.  Colloids  are  difficult  of  removal.  For 
this  reason,  where  sewage  is  to  be  treated,  turbulence  in  the 
tributary  channels  should  be  avoided. 

Alkalinity  may  indicate  the  possibility  of  success  of  the 
biologic  treatment  of  sewage,  since  bacterial  life  flourishes  better 
in  a  slightly  alkaline  than  in  a  slightly  acid  sewage.  Within 
the  normal  limits  of  the  amount  of  alkalinity  in  sewage  the  exact 
amount  has  little  significance  in  sewage  analyses.  Sewages  are 
normally  slightly  alkaline.  An  abnormal  alkalinity  or  acidity 
may  indicate  the  presence  of  certain  trades  wastes  necessitating 
special  methods  of  treatment.  A  method  of  sewage  treatment 
may  be  successful  without  changing  the  amount  of  alkalinity 
in  the  sewage  since  the  amount  of  alkalinity  is  not  inherently 
an  objection. 

Chlorine,  in  the  form  of  sodium  chloride,  is  an  inorganic  sub- 
stance found  in  the  urine  of  man  and  animals.  The  amount  of 
chlorine  above  the  normal  chlorine  content  of  pure  waters  in  the 
district  is  used  as  an  index  of  the  strength  of  the  sewage.  The 
chlorine  content  may  be  affected  by  certain  trades  wastes  such 
as  ice-cream  factories,  meat-salting  plants,  etc.,  which  will  increase 
the  amount  of  chlorine  materially.  Since  chlorine  is  an  inorganic 
substance  which  is  in  solution  it  is  not  affected  by  biological 
processes  nor  sedimentation.  Its  diminution  in  a  treatment 
plant  or  in  a  flowing  stream  is  indicative  of  dilution  and  the 
reduction  of  chlorine  will  be  approximately  proportional  to  the 
amount  of  dilution. 

Fats  have  a  recoverable  market  value  when  present  in 
sufficient  quantity  to  be  skimmed  off  the  surface  of  the  sewage. 
Ordinarily  fats  are  an  undesirable  constituent  of  sewage  as  they 
precipitate  on  and  clog  the  interstices  in  filtering  material,  they 
form  objectionable  scum  in  tanks  and  streams,  and  they  are  acted 


SIGNIFICANCE  OF  CHEMICAL  CONSTITUENTS 


359 


on  very  slowly  by  biological  processes  of  sewage  treatment. 
Although  fats  are  carbonaceous  matter  they  are  not  indicated 
by  the  oxygen  consumed  test  because  they  are  not  easily  oxidized. 
They  are  therefore  determined  in  another  manner;  by  evapora- 
tion of  the  liquid  and  extracting  the  fats  from  the  residue  by 
dissolving  them  in  ether. 

Relative  stability  and  bio-chemical  oxygen  demand  are 
the  most  important  tests  indicating  the  putrefying  character- 
istics of  sewage.  Since  stability  and  putrescibility  have  opposite 
meanings  the  relative  stability  test  is  sometimes  called  the 
putrescibility  test.  The  relative  stability  of  a  sewage  is  an 
expression  for  the  amount  of  oxygen  present  in  terms  of  the 
amount  required  for  complete  stability. 

A  relative  stability  of  75  signifies  that  the  sample 
examined  contains  a  supply  of  available  oxygen  equal 
to  75  per  cent  of  the  amount  of  oxygen  which  it  requires 
in  order  to  become  perfectly  stable.  The  available 
oxygen  is  approximately  equivalent  to  the  dissolved  oxygen 
plus  the  available  oxygen  of  nitrate  and  nitrite.1 

TABLE   72 
RELATIVE  STABILITY  NUMBERS 


Time  Required  for 
Decolorization  at 
20°  C. 
Days 

Relative 
Stability 
Number 

Time  Required 
for  Decoloriza- 
tion at  20°  C. 
Days 

Relative 
Stability 
Number 

0.5 

11 

8.0 

84 

1.0 

21 

9.0 

87 

1.5 

30 

10.0 

90 

2.0 

37 

11.0 

92 

2.5 

44 

12.0 

94 

3.0 

50 

13.0 

95 

4.0* 

60 

14.0 

96 

5.0 

68 

16.0 

97 

6.0 

75 

18.0 

98 

7.0 

80 

20.0 

99 

*  Routine  tests  are  ordinarily  incubated  for  this  period  only,  and  if  not  decolorized  in 
this  time  are  recorded  as  stable. 


Standard  Methods  of  Water  Analysis,  American  Public  Health  Asso- 
ciation, 1920. 


360          COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

The  relative  stability  numbers,  given  in  Table  72,  are  computed 
from  the  expression,  S  =  100(1  -0.7940  in  which  S  is  the 
stability  number  and  t  is  the  time  in  days  that  the  sample  has 
been  incubated  at  20°  C.  The  bio-chemical  oxygen  demand  is 
more  directly  an  index  of  the  consumption  of  available  oxygen 
by  the  biological  and  chemical  changes  which  take  place  in  the 
decomposition  of  sewage  or  polluted  water.  As  such  it  is  a 
more  valuable,  though  less  easily  performed  test  than  the  test 
of  relative  stability. 

The  methods  for  the  determination  of  the  relative  stability 
and  the  bio-chemical  oxygen  demand  are  given  to  show  more 
clearly  what  these  tests  represent.  The  procedure  in  the 
relative  stability  test  is  to  add  0.4  c.c.  of  a  standard  solution 
of  methylene  blue  to  150  c.c.  of  the  sample.  The  mixture  is  then 
allowed  to  stand  in  a  completely  filled  and  tightly  stoppered  bottle 
at  20°  C.  for  20  days  or  until  the  blue  fades  out  due  to  the  ex- 
haustion of  the  available  oxygen.  There  are  three  methods 
in  use  for  the  determination  of  the  biochemical  oxygen  demand;1 
the  relative  stability  method,  the  excess  nitrate  method,  and  the 
excess  oxygen  method  In  the  relative  stability  method  the 
sample  to  be  treated  should  have  a  relative  stability  of  at  least 
50.  If  it  is  lower  than  this  the  sample  should  be  diluted  with 
water  containing  oxygen  until  the  relative  stability  has  been 
raised  to  or  above  this  point.  The  oxygen  demand  in  parts 
per  million  is  then  expressed  as 

0'  = 


RP 

in  which  0'  is  the  oxygen  demand,  0  is  the  initial  oxygen  in  parts 
per  million  (p. p.m.)  in  the  diluting  water  or  sewage,  P  is  the 
proportion  of  sewage  in  the  mixture  expressed  as  a  ratio,  and 
R  is  the  relative  stability  of  the  mixture  expressed  as  a  decimal. 
For  the  effluents  from  sewage  treatment  plants,  polluted  waters, 
and  similar  liquids,  the  total  available  oxygen  expressed  as  the 
sum  of  the  dissolved  oxygen,  nitrites,  and  nitrates,  divided  by 

1  Determination  of  the  Biochemical  Oxygen  Demand  of  Sewage  and 
Industrial  Wastes,  by  E.  J.  Theriault,  Report  of  the  U.  S.  Public  Health 
Service,  Vol.  35,  May  7,  1920,  No.  19,  p.  1087. 

2  Standard  Methods  of  Water  Analysis,  American  Public  Health  Asso- 
ciation, 1920. 


SIGNIFICANCE  OF  CHEMICAL  CONSTITUENTS          361 

the  relative  stability  expressed  as  a  decimal  will  give  the  bio- 
chemical oxygen  demand.  The  excess  nitrate  method  requires 
the  determination  of  the  total  oxygen  available  as  dissolved 
oxygen,  nitrites,  and  nitrates  and  the  addition  of  a  sufficient 
amount  of  oxygen  in  the  form  of  sodium  nitrate  to  prevent  the 
exhaustion  of  oxygen  during  a  10-day  period  of  incubation.  At 
the  end  of  the  period  the  total  available  oxygen  is  again  deter- 
mined. The  difference  between  the  original  and  the  final  oxygen 
content  represents  the  bio-chemical  oxygen  demand.  The 
excess  oxygen  test  requires  the  determination  of  the  total  avail- 
able oxygen  as  before,  and  the  addition  of  a  sufficient  amount  of 
oxygen,  in  the  form  of  dissolved  oxygen  in  the  diluting  water, 
to  prevent  exhaustion  of  the  oxygen  in  a  10-day  period  of  incu- 
bation. The  difference  between  the  original  and  final  oxygen 
content  represents  the  bio-chemical  oxygen  demand.  Theriault 
concludes  as  a  result  of  his  tests,  that  the  relative  stability  and 
excess  nitrate  methods  are  open  to  objections  but  that  the  excess 
oxygen  method  yields  very  accurate  and  consistent  results  with 
as  little  or  less  labor  than  is  required  by  other  methods. 

Dissolved  oxygen  represents  what  its  name  implies,  the 
amount  of  oxygen  (0%)  which  is  dissolved  in  the  liquid.  Normal 
sewage  contains  no  dissolved  oxygen  unless  it  is  unusually  fresh. 
It  is  well,  if  possible,  to  treat  a  sewage  before  the  original  dis- 
solved oxygen  has  been  exhausted.  Normal  pure  surface  water 
contains  all  of  the  oxygen  which  it  is  capable  of  dissolving,  as 
shown  in  Table  73.  The  presence  of  a  smaller  amount  of  oxygen 
than  is  shown  in  this  table  indicates  the  presence  of  organic 
matter  in  the  process  of  oxidation,  which  may  be  in  such  quanti- 
ties as  ultimately  to  reduce  the  oxygen  content  to  zero.  Normal 
pure  ground  waters  may  be  deficient  in  dissolved  oxygen  because 
of  the  absence  of  available  oxygen  for  solution.  The  presence 
of  certain  oxygen-producing  organisms  in  polluted  or  otherwise 
potable  surface  waters  may  cause  a  supersaturation  with  oxygen 
however. 

The  dissolved-oxygen  test  for  polluted  water  is  probably  the 
most  significant  of  all  tests.  If  dissolved  oxygen  is  found  in  a 
polluted  water  it  means  that  putrefactive  odors  will  not  occur, 
since  putrefaction  cannot  begin  in  the  presence  of  oxygen.  It 
is  possible  for  different  strata  in  a  body  of  water  to  have  different 
quantities  of  dissolved  oxygen,  and  putrefaction  may  be  proceeding 


362 


COMPOSITION  AND  PROPERTIES  OF  SEWAGE 


in  the  lower  strata  before  the  oxygen  is  exhausted  from  the  upper 
strata.  The  oxygen  content  of  a  river  water  will  indicate  the 
ability  of  the  river  to  receive  sewage  without  resulting  in  a 
nuisance. 

TABLE  73 
SOLUBILITY  OF  OXYGEN  IN  WATER 

Under  an  atmospheric  pressure  of  760  mm.  of  mercury,  the  atmosphere 
containing  20.9  per  cent  of  oxygen. 


Temperature,  degrees  C  

0.00 

1 

2 

3 

4 

5 

6 

7 

Oxygen  in  parts  per  million  .  .  . 

14.62 

14.23 

13.84 

13.48 

13.13 

12.80 

12.48 

12.17 

Temperature,  degrees  C  

8 

9 

10 

11 

12 

13 

14 

15 

Oxygen  in  parts  per  million.  .  . 

11.87 

11.59 

11.33 

11.08 

10.83 

10.60 

10.37 

10.15 

Temperature,  degrees  C  

16 

17 

18 

19 

20 

21 

22 

23 

Oxygen  in  parts  per  million.  .  . 

9.95 

9.74 

9.54 

9.35 

9.17 

8.99 

8.83 

8.68 

Temperature   degrees  C 

24 

25 

26 

27 

28 

29 

30 

Oxygen  in  parts  per  million.  .  . 

8.53 

8.38 

8.22 

8.07 

7.92 

7.77 

7.63 

211.  Sewage  Bacteria. — A  slight  knowledge  of  the  nature 
of  bacteria  is  necessary  in  order  that  the  biological  changes 
which  occur  in  the  treatment  of  sewage  may  be  understood. 
Bacteria  are  living  organisms  which  are  so  small  that  it  is  difficult 
or  impossible  to  study  them  either  with  the  eye  alone  or  with 
the  aid  of  powerful  microscopes.  They  are  studied  by  means 
of  cultures,  stains,  and  certain  characteristic  phenomena  such 
as  the  production  of  a  gas,  the  production  of  a  red  colony  on 
litmus  lactose  agar,  etc.  Bacteria  occur  in  three  forms: 
spherical,  called  coccus;  cylindrical,  called  bacillus;  and  spiral, 
called  spirillum.  In  size  they  vary  from  the  largest  at  about 
1/10,000  of  an  inch  to  sizes  so  small  as  to  be  invisible  under  the 
most  powerful  microscope.  An  ordinary  size  is  1/25,000  of  an 
inch.  The  cylindrical  or  rod  bacteria  are  about  four  times  as  long 
as  they  are  wide.  Some  bacteria  possess  the  power  of  motion 
due  to  the  presence  of  flagella  or  hairs  which  can  be  moved  and 


ORGANIC  LIFE  IN  SEWAGE  363 

cause  the  cell  to  progress  at  a  rate  as  high  as  18  cm.  per  hour, 
but  usually  the  rate  is  very  much  less  than  this.  The  compo- 
sition of  the  bacterial  cell  has  never  been  definitely  determined. 

Bacteria  are  unicellular  plants.  They  possess  no  digestive 
organs  and  apparently  obtain  their  food  by  absorption  from  the 
surrounding  media.  Reproduction  is  by  the  division  of  the  cell 
into  two  approximately  equal  portions.  This  reproduction  may 
occur  as  frequently  as  once  every  half  hour  and  if  unchecked 
would  quickly  mount  to  unimaginable  numbers.  The  natural 
cause  limiting  the  growth  of  bacteria  is  the  generation  by  the 
bacterium  of  certain  substances  such  as  the  amino  acids,  which 
are  injurious  to  cell  life.  The  exhaustion  of  the  food  supply  is 
not  considered  as  an  important  cause  of  inhibition  of  multipli- 
cation. The  products  of  growth  of  one  species  of  bacteria  may  be 
helpful  or  harmful  to  other  forms.  Where  the  products  are 
helpful  the  effect  is  known  as  symbiosis,  and  where  harmful  the 
effect  is  known  as  antibiosis.  In  sewage  the  presence  of  both 
aerobic  and  anaerobic  bacteria  is  usually  mutually  helpful  and 
the  condition  is  an  example  of  symbiosis.  The  aerobes,  some- 
times called  obligatory  aerobes,  are  bacteria  which  demand 
available  oxygen  for  their  growth.  The  anaerobes,  or  obligatory 
anaerobes,  can  grow  only  in  the  absence  of  oxygen.  There  are 
other  forms  that  are  known  as  facultative  anaerobes  (or  aerobes) 
whose  growth  is  independent  of  the  presence  or  absence  of 
oxygen. 

Spores  are  formed  by  some  bacteria  when  they  are  subjected 
to  an  unfavorable  environment  such  as  high  temperatures,  the 
absence  of  food,  the  absence  of  moisture,  etc.  Spores  are  cells 
in  which  growth  and  animation  are  suspended  but  the  life  of 
the  cell  is  carried  on  through  the  unsuitable  period,  somewhat 
similar  to  the  condition  in  a  plant  seed. 

212.  Organic  Life  in  Sewage. — Living  organisms,  both  plants 
and  animals,  exist  in  sewage.  Bacteria  are  the  smallest  of  these 
organisms.  Others,  which  can  be  studied  easily  under  the 
microscope  or  can  be  seen  with  difficulty  by  the  naked  eye  but 
which  do  not  require  special  cultures  for  their  study,  are  classed 
as  microscopic  organisms  or  plankton.  Organisms  which  are 
large  enough  to  be  studied  without  the  aid  of  a  microscope  or 
special  cultures  are  classed  as  macroscopic.  The  part  taken  in 
the  biolysis  of  sewage  by  macroscopic  organisms  belonging  to 


364          COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

the  animal  kingdom,  such  as  birds,  fish,  insects,  rodents,  etc., 
which  feed  upon  substances  in  the  sewage  is  so  inconsequential 
as  to  be  of  no  importance.  Both  plants  and  animals  are  found 
among  the  macroscopic  organisms. 

Organisms  in  sewage  may  be  either  harmful,  harmless,  or 
beneficial.  From  the  viewpoint  of  mankind  the  harmful  organ- 
isms are  the  pathogenic  bacteria.  Their  condition  of  life  in  sewage 
is  not  normal  and  in  general  their  existence  therein  is  of  short 
duration.  It  may  be  of  sufficient  length,  however,  to  permit 
the  transmission  of  disease.  The  diseases  which  can  be  trans- 
mitted by  sewage  are  only  those  that  are  contracted  through 
the  alimentary  canal,  such  as  typhoid  fever,  dysentery, 
cholera,  etc.  Diseases  are  not  commonly  contracted  by  contact 
of  sewage  with  the  skin  nor  by  breathing  the  air  of  sewers.  It 
is  safe  to  work  in  and  around  sewage  so  long  as  the  sewage  is 
kept  out  of  the  mouth,  and  asphyxiating  or  toxic  gases  are 
avoided. 

The  beneficial  organisms  in  sewage  are  those  on  which 
dependence  is  placed  for  the  success  of  certain  methods  of  treat- 
ment. These  organisms  have  not  all  been  isolated  or  identified. 

The  total  number  of  bacteria  in  a  sample  of  sewage  has  little 
or  no  significance.  In  a  normal  sewage  the .  number  may  be 
between  2,000,000  and  20,000,000  per  c.c.  and  because  of  the 
extreme  rapidity  of  multiplication  of  bacteria  a  sample  showing 
a  count  of  1,000,000  per  c.c.  on  the  first  analysis  may  show  4  to 
5  times  as  many  3  or  4  hours  later.  A  bacterial  analysis  of 
sewage  is  ordinarily  of  little  or  no  value,  since  pathogenic  organ- 
isms are  practically  certain  to  be  present,  there  is  no  interest 
in  the  harmless  organisms,  and  the  helpful  nitrifying  and  aerobic 
bacteria  will  not  grow  on  ordinary  laboratory  media.  Occa- 
sionally the  presence  of  certain  bacteria  may  indicate  the  presence 
of  certain  trades  wastes.  In  general,  the  total  bacterial  count, 
as  sometimes  reported,  represents  only  the  number  of  bacteria 
which  have  grown  under  the  conditions  provided.  It  bears  no 
relation  to  the  total  number  of  bacteria  in  the  sample. 

The  presence  of  bacteria  in  sewage  is  of  great  importance 
however,  as  practically  all  methods  of  treatment  depend  on 
bacterial  action,  and  all  sewages  which  do  not  contain  deleterious 
trades  wastes,  contain  or  will  support  the  necessary  bacteria 
for  their  successful  treatment,  if  properly  developed. 


DECOMPOSITION  OF  SEWAGE  365 

213.  Decomposition  of  Sewage. — If  a  glass  container  be  filled 
with  sewage  and  allowed  to  stand,  open  to  the  air,  a  black  sedi- 
ment will  appear  after  a  short  time,  a  greasy  scum  may  rise  to 
the  surface,  and  offensive  odors  will  be  given  off.  This  con- 
dition will  persist  for  several  weeks,  after  which  the  liquid  will  be- 
come clear  and  odorless.  The  sewage  has  been  decomposed  and  is 
now  in  a  stable  condition.  The  decomposition  of  sewage  is  brought 
about  by  bacterial  action  the  exact  nature  of  which  is  uncertain. 

It 1  is  well  established  that  many  of  the  chemical 
effects  wrought  by  bacteria,  as  by  other  living  cells,  are 
due,  not  to  the  direct  action  of  the  protoplasm,  but  to 
the  intervention  of  soluble  ferments  or  enzymes. 

Enzymes  are  soluble  ferments  produced  by  the  growth  of  the 
bacterial  cell. 

In 2  many  cases  the  enzymes  diffuse  out  from  the 
cell  and  exert  their  effort  on  the  ambient  substances  .  .  . 
in  others  the  enzyme  action  occurs  within  the  cell  and 
the  products  pass  out,  (for  example)  .  .  .  the  alcohol- 
producing  enzymes  of  the  yeast  cell  act  upon  sugar  within 
the  cell,  the  resulting  alcohol  and  carbon  dioxide  being 
ejected. 

Other  chemical  effects  may  be  brought  about  by  the  direct  action 
of  the  living  cells,  but  this  has  never  been  well  established. 

Metabolism  is  the  life  process  of  living  cells  by  which  they 
absorb  their  food  and  convert  it  into  energy  and  other  products. 
It  is  the  metabolism  of  bacterial  growth  that  in  itself  or  by  the 
production  of  enzymes  hastens  the  putrefactive  or  oxidizing 
stages  of  the  organic  cycles  in  sewage  treatment.  Bacteria  can 
assimilate  only  liquid  food  since  they  have  no  digestive  tract 
through  which  solid  food  can  enter.  The  surrounding  solids 
are  dissolved  by  the  action  of  the  enzymes,  the  resulting  solution 
diffusing  through  the  chromatin  or  outer  skin,  and  being  digested 
throughout  the  interior  cytoplasm. 

Bacteria  are  sometimes  classified  as  parasites  and  saprophytes. 
The  parasites  live  only  on  the  growing  cells  of  other  plant  or 
animal  life.  The  saprophytes  obtain  their  food  only  from  the 

1  Jordan,  General  Bacteriology,  1909,  p.  91. 

2  Ibid. 


366          COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

life  products  of  living  organisms  and  do  not  exist  at  the  expense 
of  the  organisms  themselves.  Facultative  saprophytes  (or 
parasites)  may  exist  on  either  living  or  dead  tissue. 

The  decomposition  of  sewage  may  be  divided  into  anaerobic 
and  aerobic  stages.  These  conditions  are  usually,  but  not 
always,  distinctly  separate.  The  growth  of  certain  forms  of 
bacteria  is  concurrent,  while  the  growth  of  other  forms  is  dependent 
on  the  results  of  the  life  processes  of  other  bacteria  in  the  early 
stages  of  decomposition. 

When  sewage  is  very  fresh  it  contains  some  oxygen.  This 
oxygen  is  quickly  exhausted  so  that  the  first  important  step  in 
the  decomposition  of  sewage  is  carried  on  under  anaerobic  condi- 
tions. This  is  accompanied  by  the  creation  of  foul  odors  of 
organic  substances,  ammonia,  hydrogen  sulphide,  etc.;  other 
odorless  gases  such  as  carbon  dioxide,  hydrogen,  and  marsh 
gas,  the  latter  being  inflammable  and  explosive;  and  other  com- 
plicated compounds.  An  exception  to  the  rule  that  putrefac- 
tion takes  place  only  in  the  absence  of  oxygen  is  the  production 
of  other  foul-smelling  substances  by  the  putrefactive  activity 
of  obligatory  and  facultative  aerobes.  Hydrogen  sulphide  may 
be  produced  apparently  in  the  presence  of  oxygen  the  action 
which  takes  place  not  being  thoroughly  understood. 

The  biolysis  of  sewage  is  the  term  applied  to  the  changes 
through  which  its  organic  constituents  pass  due  to  the  metab- 
olism of  bacterial  life.  Organic  matter  is  composed  almost 
exclusively  of  the  four  elements:  carbon,  oxygen,  hydrogen, 
and  nitrogen  (COHN)  and  sometimes  in  addition  sulphur  and 
phosphorus.  The  organic  constituents  of  sewage  can  be  divided 
into  the  proteins,  carbohydrates,  and  fats.  The  proteins  are 
principally  constituents  of  animal  tissue,  but  they  are  also  found 
in  the  seeds  of  plants.  The  principal  distinguishing  character- 
istic of  the  proteins  is  the  possession  of  between  15  and  16  per  cent 
of  nitrogen.  To  this  group  belong  the  albumens  and  casein. 
The  carbohydrates  are  organic  compounds  in  which  the  ratio 
of  hydrogen  to  oxygen  is  the  same  as  in  water,  and  the  number 
of  carbon  atoms  is  6  or  a  multiple  of  6.  To  this  group  belong 
the  sugars,  starches  and  celluloses.  The  fats  are  salts  formed, 
together  with  water,  by  the  combination  of  the  fatty  acids  with 
the  tri-acid  base  glycerol.  The  more  common  fats  are  stearin, 
palmatin,  olein,  and  butyrine.  The  soaps  are  mineral  salts  of  the 


THE  NITROGEN  CYCLE  367 

fatty  acids  formed  by  replacing  the  weak  base  glycerol  with  some 
of  the  stronger  alkalies. 

The  first  state  in  the  biolysis  of  sewage  is  marked  by  the 
rapid  disappearance  of  the  available  oxygen  present  in  the  water 
mixed  with  organic  matter  to  form  sewage.  In  this  state  the 
urea,  ammonia,  and  other  products  of  digestive  or  putrefactive 
decomposition  are  partially  oxidized  and  in  this  oxidation  the 
available  oxygen  present  is  rapidly  consumed,  the  conditions 
in  the  sewage  becoming  anaerobic.  The  second  state  is  putre- 
faction in  which  the  action  is  under  anaerobic  conditions.  The 
proteins  are  broken  down  to  form  urea,  ammonia,  the  foul- 
smelling  mercaptans,  hydrogen  sulphide,  etc.,  and  fatty  and 
aromatic  acids.  The  carbohydrates  are  broken  down  into  their 
original  fatty  acid,  water,  carbon  dioxide,  hydrogen,  methane, 
and  other  substances.  Cellulose  is  also  broken  down  but  much 
more  slowly.  The  fats  and  soaps  are  affected  somewhat  similarly 
to  the  hydrocarbons  and  are  broken  down  to  form  the  original 
acids  of  their  make  up  together  with  carbon  dioxide,  hydrogen, 
methane,  etc.  The  bacterial  action  on  facts  and  soaps  is  much 
slower  than  on  the  proteins,  and  the  active  biological  agents 
in  the  biolysis  of  the  hydrocarbons,  fats,  and  soaps  are  not  so 
closely  confined  to  anaerobes  as  in  the  biolysis  of  the  proteins. 
The  third  state  in  the  biolysis  of  sewage  is  the  oxidation  or 
nitrification  of  the  products  of  decomposition  resulting  from 
the  putrefactive  state.  The  products  of  decomposition  are 
converted  to  nitrites  and  nitrates,  which  are  in  a  stable  condition 
and  are  available  for  plant  food.  It  must  be  understood  that 
the  various  states  may  be  coexistent  but  that  the  conditions 
of  the  different  states  predominate  approximately  in  the  order 
stated.  In  the  biolysis  of  sewage  there  is  no  destruction  of  matter. 
The  same  elements  exist  in  the  same  amount  as  at  the  start  of  the 
biolytic  action. 

214.  The  Nitrogen  Cycle. — Nitrogen  is  an  element  that  is 
found  in  all  organic  compounds.  Its  presence  is  necessary  to  all 
plant  and  animal  life.  The  nitrogenous  compounds  are  most 
readily  attacked  by  bacterial  action  in  sewage  treatment.  The 
non-nitrogenous  substances  such  as  soaps  and  fats,  and  the  inor- 
ganic compounds  are  more  slowly  affected  by  bacterial  action 
alone.  The  element  nitrogen  passes  through  a  course  of  events 
from  life  to  death  and  back  to  life  again  that  is  known  as  the 


368        COMPOSITION  AND  PROPERTIES  OF  SEWAGE 

Nitrogen  Cycle.     It  is  typical  of  the  cycles  through  which  all 
of  the  organic  elements  pass. 

Upon  the  death  of  a  plant  or  animal,  decomposition  sets  in 
accompanied  by  the  formation  of  urea  which  is  broken  down 
into  ammonia.  This  is  known  as  the  putrefactive  stage  of  the 
Nitrogen  Cycle.  The  next  state  is  nitrification  in  which  the 
compounds  of  ammonia  are  oxidized  to  nitrites  and  nitrates, 
and  are  thus  prepared  for  plant  food.  In  the  state  of  plant  life 
the  nitrites  and  nitrates  are  denitrified  so  as  to  be  available  as  a 
plant  or  animal  food.  The  highest  state  of  the  Nitrogen  Cycle 
is  animal  life,  in  which  nitrogen  is  a  part  of  the  living  animal 
substance  or  is  charged  from  protein  to  urea,  ammonia,  etc.,  by 
the  functions  of  life  in  the  animal.  Upon  the  death  of  this 
animal  organism  the  cycle  is  repeated.  The  Nitrogen  Cycle, 
like  the  cycle  of  Life  and  Death,  is  purely  an  ideal  condition  as 
in  nature  there  are  many  short  circuits  and  back  currents  which 
prevent  the  continuous  progression  of  the  cycle.  The  con- 
ception of  this  cycle  is  an  aid,  however,  in  understanding  the 
processes  of  sewage  treatment. 

215.  Plankton    and    Macroscopic    Organisms. — In    general 
the  part  played  by  these  organisms  in  the  biolysis  of  sewage  is 
not  sufficiently  well  understood  to  aid  in  the  selection  of  methods 
of  sewage  treatment  involving  their  activities.    The  presence 
in  bodies  of  water  receiving  sewage,  of  certain  plankton  which 
are  known  to  exist  only  when  putrefaction  is  not  imminent, 
indicates  that  the  body  of  water  into  which  the  discharge  of 
sewage  is  occurring  is  not  being  overtaxed.    The  control  of  sewage 
treatment  plant  effluents  so  as  to  avoid  the  poisoning  of  fish  life 

.or  the  contamination  of  shell  fish  is  likewise  important.  The 
study  of  plankton  and  macroscopic  life  in  the  treatment  of  sewage 
is  an  open  field  for  research. 

216.  Variations  in  the  Quality  of  Sewage. — The  quality  of 
sewage  varies  with  the  hour  of  the  day  and  the  season  of  the 
year.     Some  of  the  causes  of  these  variations  are:   changes  in  the 
amount  of  diluting  water  due  to  the  inflow  of  storm  water  or 
flushing  of  the  streets  or  sewers;  variations  in  domestic  activities 
such  as  the  suspension  of  contributions  of  organic  wastes  during 
the   night,    Monday's   wash,    etc.;     characteristics   of   different 
industries  which  discharge  different  kinds  of  wastes  according 
to  the   stage  of  the  manufacturing  process,   etc.     In  general 


VARIATIONS  IN  THE  QUALITY  OF  SEWAGE 


369 


!  TABLE  74 

SEWAGE  ANALYSES   SHOWING  HOURLY,  DAILY,  AND  SEASONAL  VARIATIONS 

IN  QUALITY 


Place 

Time 

Total 
Nitro- 
gen 

Chlo- 
rine 

Sus- 
pended 
Matter 

Remarks 

Refer- 
ence 

Marion    Ohio    . 

Mid't-noon,  5-21-06. 

45 

53 

190 

Industrial 

1 

Noon-mid't  5-21-06. 

37 

94 

133 

Domestic 

1 

Westerville,  Ohio  

Day 

10.2 

76 

118 

f  college 

1 

Night 

2.6 

74 

41 

\  town 

1 

Columbus,  Ohio  

1904-1905 

Mid't  to  2  a.m. 

4.6 

50 

131 

2 

2  a.m.  to    4  a.m. 

3.0 

52 

95 

2 

4  a.m.  to    6  a.m. 

2.3 

51 

83 

2 

6  a.m.  to    8  a.m. 

2.7 

48 

83 

2 

8  a.m.  to  10  a.m. 

16.3 

66 

476 

2 

10  a.m.  to  noon 

11.4 

100 

324 

2 

Noon  to  2  p.m. 

11.3 

86 

246 

2 

2  p.m.  to    4  p.m. 

12.3 

78 

246 

2 

4  p.m.  to    6  p.m. 

22.0 

78 

368 

2 

6  p.m.  to    8  p.m. 

8.2 

71 

209 

2 

8  p.m.  to  10  p.m. 

7.8 

80 

120 

2 

10  p.m.  to  mid't 

6.2 

56 

117 

2 

Center  Ave.,  Chicago. 

Mid't  to    3  a.m. 

123 

3 

4  a.m.  to    7  a.m. 

316 

3 

8  a.m.  to  11  a.m. 

608 

3 

Noon  to  3  p.m. 

785 

3 

4  p.m.  to    7  p.m. 

717 

3 

8  p.m.  to  11  p.m. 

287 

3 

Columbus,  Ohio  

Sunday 

6.7 

55 

858 

2 

Monday 

9.1 

66 

1048 

2 

Tuesday 

9.4 

69 

1024 

2 

Wednesday 

9.6 

68 

1005 

2 

Thursday 

9.2 

66 

990 

2 

Friday 

9.2 

67 

1018 

2 

Saturday 

9.3 

67 

1016 

2 

Baltimore,  1907-1908. 

Aug.  1  to  Sept.  1 

16.0 

246 

4 

Sept.    4  to  Oct.     3 

19.0 

190 

4 

Oct.      6  to  Nov.    4 

20.0 

188 

4 

Nov.  15  to  Nov.  29 

20.0 

164 

4 

Dec.     3  to  Dec.  29 

20.0 



123 

4 

Jan.      6  to  Jan.    21 

19.0 

127 

4 

Feb.     2  to  Feb.  26 

20.0 

149 

4 

Feb.   29  to  Mar.  24 

28.0 



274 

4 

Mar.  27  to  April  29 

25.0 



165 

4 

April  30  to  May  26 

19.0 

104 

4 

June     8  to  July   11 

15.0 



88 

4 

July    13  to  Aug.     8 

9.5 



124 

4 

References: 

1.  1908  Report  of  the  Ohio  State  Board  of  Health. 

2.  Report  on  Sewage  Purification  at  Columbus,  Ohio,  by  G.  A.  Johnson,  1905. 

3.  Report  on  Industrial  Wastes  from  the  Stock  Yards  and  Packingtown  in  Chicago,  by 

the  Sanitary  District  of  Chicago.  1921. 

4.  Report  of  the  Baltimore  Sewerage  Commission,  1911. 


370          COMPOSITION  AND   PROPERTIES  OF  SEWAGE 

night  sewage  is  markedly  weaker  than  day  sewage  in  both  domestic 
and  industrial  wastes,  but  in  specific  cases  the  varying  strength 
depends  entirely  upon  the  characteristics  of  the  district.  Some 
analyses  are  given  in  Table  74,  which  emphasize  these 
points. 

217.  Sewage  Disposal. — Previous  to  the  development  of  the 
water-carriage  method  for  removing  human  excreta  and  other 
liquid  wastes  the  solid  matter  was  disposed  of  by  burial  and  the 
liquid  wastes  were  allowed  to  seep  into  the  ground  or  to  run 
away  over  its  surface.     Following  the  development  of  the  water- 
carriage  system,  which  necessitated  the  development  of  sewers, 
the  problem  of  ultimate  disposal  was  rendered  more  serious  by 
the  concentration  of  human  excreta  together  with  a  large  volume 
of  water.     The  unthinking  citizen  believes  the  problem  of  sewage 
disposal  is  solved  when  the  toilet  is  flushed  or  the  bath  tub  is 
drained.     The  problem  may  more  truly  be  said  to  commence 
at  this  point. 

It  would  appear  that  the  simplest  method  of  disposal  of 
sewage  would  be  to  discharge  it  into  the  nearest  water  course. 
Unfortunately  the  nature  of  sewage  is  such  that  it  may  be  either 
highly  offensive  to  the  senses  or  dangerous  to  health  or  both, 
when  discharged  in  this  manner.  Only  the  most  fortunate 
communities  are  favored  with  a  body  of  water  of  sufficient  size 
to  receive  sewage  without  creating  a  nuisance. 

The  problems  of  sewage  disposal  are  to  prevent  nuisances 
causing  offense  to  sight  and  smell;  to  prevent  the  clogging  of 
channels;  to  protect  pumping  machinery;  to  protect  public 
water  supplies;  to  protect  fish  life;  to  prevent  the  contamination 
of  shell  fish;  to  recover  valuable  constituents  of  the  sewage; 
to  enrich  and  to  irrigate  the  soil;  to  safeguard  bathing  and  boat- 
ing ;  for  other  minor  purposes ;  and  in  some  cases  to  comply  with 
the  law.  Sewage  may  be  treated  to  attain  one  or  more  of  these 
objects  by  methods  of  treatment  varying  as  widely  as  the  objects 
to  be  attained. 

218.  Methods  of  Sewage  Treatment. — In  studying  the  sub- 
ject of  sewage  treatment  it  must  be  borne  in  mind  that  it  is 
impossible  to  destroy  any  of  the  elements  present.     They  may  be 
removed   from   the   mixture   only   by   gasification,    straining   or 
sedimentation.     Their  chemical  combinations  may  be  so  changed, 
however,  as  to  result  in  different  substances  than  those  intro- 


METHODS  OF  SEWAGE   TREATMENT  371 

duced  to  the  treatment  plant.     It  is  with  these  chemical  changes 
that  the  student  of  sewage  treatment  is  interested. 

The  methods  of  sewage  treatment  can  be  classified  as  mechan- 
ical, chemical  and  biological.  These  classifications  are  not  sepa- 
rated by  rigid  lines  but  may  overlap  in  certain  treatment  devices 
or  methods.  Mechanical  methods  of  treatment  are  exemplified 
by  sedimentation,  and  screening.  Chemical  precipitation  and 
sterilization  are  examples  of  chemical  methods.  The  biological 
methods,  the  most  important  of  all,  include  dilution,  septiciza- 
tion,  filtration,  sewage  farming,  activated  sludge,  etc.  If  for 
any  reason  it  is  desired  to  treat  sewage  by  more  than  one  of  these 
methods  the  procedure  should  follow  as  nearly  as  possible  the 
order  of  the  occurrence  of  the  phenomena  in  the  natural  biolysis 
of  sewage.  For  example,  in  one  treatment  plant  the  sewage 
would  first  pass  through  a  grit  chamber  where  the  coarse  sediment 
would  be  removed,  then  through  a  screen  where  the  floating 
matter  and  coarse  suspended  matter  would  be  removed,  then 
to  a  sedimentation  basin  where  some  finer  suspended  matter 
might  settle  out,  then  to  a  digestive  tank  where  the  solid  matter 
deposited  would  be  worked  upon  by  bacterial  action  and  partially 
liquefied.  Simultaneous  to  the  liquefaction  of  the  deposited 
solid  matter  the  liquid  effluent  from  the  digestive  tank  might 
proceed  to  an  aerating  device  to  expedite  oxidation,  then  to  an 
aerobic  filter,  and  finally  to  disposal  by  dilution. 


CHAPTER  XIV 
DISPOSAL  BY  DILUTION 

219.  Definition. — Disposal  of  sewage  by  dilution  is  the  dis- 
charge of  raw  sewage  or  the  effluent  from  a  treatment  plant  into 
a  body  of  water  of  sufficient  size  to  prevent  offense  to  the  senses 
of  sight  and  smell,  and  to  avoid  danger  to  the  public  health. 

220.  Conditions  Required  for  Success. — Among  the  desired 
conditions   for   successful   disposal   by    dilution   are:     adequate 
currents   to   prevent    sedimentation   and   to    carry   the    sewage 
away  from  all  habitations  before  putrefaction  sets  in,  or  sufficient 
diluting  water  high  in  dissolved  oxygen  to  prevent  putrefaction; 
a  fresh  or  non-septic   sewage;     absence  of  floating  or  rapidly 
settling  solids,  grease  or  oil;     and  absence  of  back  eddies  or 
quiet  pools  favorable  to  sedimentation  in  the  stream  into  which 
disposal  is  taking  place.     The  conditions  which  should  be  pre- 
vented are:  offensive  odors  due  to  sludge  banks,  the  rise  of  septic 
gases,   and   unsightly   floating   or   suspended   matter.     In   some 
instances  the  pollution  of  the  receiving  body  of  water  is  undesirable 
and  the  sewage  must  be  freed  from  pathogenic  organisms  and  the 
danger  of  aftergrowths  minimized  before  disposal.     Such   con- 
ditions are  typified  at  Baltimore,  where  the  sewage  is  discharged 
into  Back  Bay,  an  arm  of  Chesapeake  Bay.     One  of  the  impor- 
tant industries  of  the  state  of  Maryland  is  the  cultivation  of  oysters. 
The  pollution  of  the  Bay  was  therefore  so  objectionable  that  care- 
ful treatment  of  the  Baltimore  sewage  has  been  a  necessary 
preliminary  to  final  disposal  by  dilution.     It  is  unwise  to  draw 
public  water  supplies,  without  treatment,  from  a  stream  receiving 
a  sewage  effluent,  no  matter  how  careful  or  thorough  the  treat- 
ment of  the  sewage.     The  treatment  of  the  sewage  is  a  safe- 
guard, and  lightens  the  load  on  the  water  purification  plant, 
but  under  no  considerations  can  it  be  depended  upon  to  protect 
the  community  consuming  the  diluted  effluent. 

372 


SELF-PURIFICATION  OF  RUNNING  STREAMS  373 

The  sewer  outlet  should  be  located  well  out  in  the  current  of 
the  stream,  lake,  or  harbor.  Deeply  submerged  outlets  are 
usually  better  than  an  outlet  at  the  surface,  as  a  better  mixture 
of  the  sewage  and  water  is  obtained.  The  discharge  of  sewage 
into  a  body  of  water  of  which  the  surface  level  changes,  alternately 
covering  and  exposing  large  areas  of  the  bottom  is  unwise,  as  the 
ludge  which  is  deposited  during  inundation  will  cause  offensive 
odors  when  uncovered.  Such  conditions  must  be  carefully 
guarded  against  when  selecting  a  point  of  disposal  in  tidal 
estuaries  because  of  the  frequent  fluctuations  in  level. 

221.  Self-Purification  of  Running  Streams. — The  self-purifi- 
cation of  running  streams  is  due  to  dilution,  sedimentation,  and 
oxidation.  The  action  is  physical,  chemical,  and  biological. 
When  putrescible  organic  matter  is  discharged  into  water  the 
offensive  character  of  the  organic  matter  is  minimized  by  dilution. 
If  the  dilution  is  sufficiently  great,  it  alone  may  be  sufficient  to 
prevent  all  nuisance.  The  oxidation  of  the  organic  matter 
commences  immediately  on  its  discharge  into  the  diluting  water 
due  to  the  growth  and  activity  of  nitrifying  and  other  oxidizing 
organisms  and  to  a  slight  degree  to  direct  chemical  reaction. 
So  long  as  there  is  sufficient  oxygen  present  in  the  water  septic 
conditions  will  not  exist  and  offensive  odors  will  be  absent. 
When  the  organic  matter  is  completely  nitrified  or  oxidized 
there  will  be  no  further  demand  on  the  oxygen  content  of  the 
stream  and  the  stream  will  be  said  to  have  purified  itself.  At 
the  same  time  that  this  oxidation  is  going  on  some  of  the  organic 
matter  will  be  settling  due  to  the  action  of  sedimentation.  If 
oxidation  is  completed  before  the  matter  has  settled  on  the 
bottom  the  result  will  be  an  inoffensive  silting  up  of  the  river. 
If  oxidation  is  not  complete,  however,  the  result  will  be  offensive 
putrefying  sludge  banks  which  may  send  their  stinks  up  through 
the  superimposed  layers  of  clean  water  to  pollute  the  surrounding 
atmosphere. 

The  most  important  condition  for  the  successful  self-purifi- 
cation of  a  stream  is  an  initial  quantity  of  dissolved  oxygen  to 
oxidize  all  of  the  organic  matter  contributed  to  it,  or  the  addition 
of  sufficient  oxygen  subsequent  to  the  contribution  of  sewage  to 
complete  the  oxidation.  Oxygen  may  be  added  through  the  dilu- 
tion received  from  tributaries,  through  aeration  over  falls  and 
rapids,  or  by  quiescent  absorption  from  the  atmosphere.  The 


374  DISPOSAL  BY  DILUTION 

rapidity  of  self-purification  is  dependent  on  the  character  of  the 
organic  matter,,  the  presence  of  available  oxygen,  the  rate  of 
reaeration,  temperature,  sedimentation,  and  the  velocity  of  the 
current.  Sluggish  streams  are  more  likely  to  purify  themselves 
in  a  shorter  distance  and  rapidly  flowing  turbulent  streams 
are  more  likely  to  purify  themselves  in  a  shorter  time,  other 
conditions  being  equal.  Although  the  absorption  of  oxygen  by 
a  stream  whose  surface  is  broken  is  more  rapid  than  through  a 
smooth  unbroken  surface,  the  growth  of  algae,  biological  activity, 
the  effect  of  sunlight,  and  sedimentation  are  more  potent  factors 
and  have  a  greater  effect  in  sluggish  streams  than  the  slightly 
more  rapid  absorption  of  oxygen  in  a  turbulent  stream.  It  is 
frequently  more  advantageous  to  discharge  sewage  into  a 
swiftly  moving  stream,  however,  regardless  of  the  conditions 
of  self -purification,  as  the  undesirable  conditions  which  may 
result  occur  far  from  the  point  of  disposal  and  may  be  offensive 
to  no  one. 

The  sewage  from  a  population  of  about  3,000,000  persons 
residing  in  and  about  Chicago  is  discharged  into  the  Chicago 
Drainage  Canal.  It  ultimately  reaches  tide  water  through 
the  Des  Plaines,  the  Illinois,  and  the  Mississippi  Rivers.  The 
action  occurring  in  these  channels  is  one  of  the  best  illustrations 
known  of  the  self-purification  of  a  stream.  In  Table  75  are 
shown  the  results  of  analyses  of  samples  taken  at  various  points 
below  the  mouth  of  the  Chicago  River  where  the  diluting  water 
from  Lake  Michigan  enters,  to  Graf  ton,  Illinois,  at  the  junction 
of  the  Illinois  and  Mississippi  Rivers  about  40  miles  above  St. 
Louis.  The  effect  of  the  physical  characteristics  of  the  stream 
on  its  chemical  composition  is  well  illustrated  in  this  table. 
The  rise  in  the  chlorine  content  between  Lake  Michigan  and  the 
entrance  to  the  Drainage  Canal  is  a  measure  of  the  addition 
of  sewage.  Since  the  chlorine  is  an  inorganic  substance  which 
is  not  affected  by  biologic  action,  its  loss  in  concentration  in  the 
lower  reaches  of  the  rivers  is  due  to  dilution  by  tributaries  and 
sedimentation,  e.g.,  between  the  end  of  the  canal  at  Lockport 
and  the  sampling  point  at  Joliet,  the  entrance  of  the  Des  Plaines 
River  reduces  the  concentration  of  chlorine  from  124.5  to  41.5 
parts  per  million.  The  entrance  of  the  Kankakee  River  at 
Dresden  Heights  further  reduces  the  chlorine  to  24.5  p.p.m. 
The  increase  of  albuminoid  and  ammonia  nitrogen  accompanied 


SELF-PURIFICATION  OF  RUNNING  STREAMS 


375 


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376  DISPOSAL  BY  DILUTION 

by  a  decrease  in  nitrites  and  nitrates,  between  the  upper  end 
of  the  canal  at  Bridgeport  and  its  lower  end  at  Lockport  indicates 
the  reducing  action  proceeding  therein.  The  oxidizing  action 
over  the  various  dams  and  the  effect  of  dilution  with  water 
containing  oxygen  is  shown  between  miles  34  and  38,  at  mile  79, 
and  at  mile  294.  The  excellent  effect  of  quiescent  sedimentation 
and  aeration  in  Peoria  Lakes  is  shown  between  miles  145,  161  and 
165. 

222.  Self-Purification  of  Lakes. — Sewage  may  be  disposed  of 
into  lakes  with  as  great  success  as  into  running  streams  if  condi- 
tions exist  which  are  favorable  to  self -purification.     Lakes  and 
rivers  purify  themselves  from  the  same  causes;    oxidation,  sedi- 
mentation, etc.,  but  in  the  former  the  currents  are  much  less 
pronounced    and    may    be    entirely    absent.     In    shallow    lakes 
(20  feet  or  less  in  depth)  dependence  must  be  placed  on  horizontal 
currents  and  the  stirring  action  of  the  wind  to  keep  the  water 
in  motion  in  order  that  the  sewage  and  the  diluting  water  may 
be  mixed.     In  deeper  bodies  of  water,  currents  induced  by  the 
wind  are  helpful  but  entire  dependence  need  not  be  placed  upon 
them.     Vertical  currents,  and  the  seasonal  turnovers  in  the  spring 
and  fall  completely  mix  the  waters  of  the  lake  above  those  layers 
of  water  whose  temperature  never  rises  higher  than  4°  C. 

In  the  early  winter  the  cold  air  cools  the  surface  waters  of 
a  lake.  The  cooling  increases  the  density  of  the  surface  water 
causing  it  to  sink,  and  allowing  the  warmer  layers  below  to  rise 
and  become  cooled.  After  the  temperature  of  the  entire  lake 
has  reached  4°  C.  the  vertical  currents  induced  by  temperature 
cease,  as  continued  cooling  decreases  the  density  of  the  surface 
water  maintaining  the  same  layer  at  the  surface.  In  the  spring 
as  the  temperature  of  the  surface  water  rises  to  4°  C.  and  above 
it  becomes  heavier  and  drops  through  the  colder  water  below 
causing  vertical  currents.  These  phenomena  are  known  as  the 
fall  and  spring  turnovers.  The  former  is  more  pronounced. 
These  turnovers  are  effective  in  assisting  in  the  self-purification 
of  lakes. 

223.  Dilution  in  Salt  Water. — The  oxygen  content    in  salt 
water  is  about  20  per  cent  less  than  in  fresh  water  at  the  same 
temperature.     The    greater    content    of   matter   in    solution   in 
salt  water  reduces  its  capacity  to  absorb  many  sewage  solids. 
This,  together  with  the  chemical  reaction  between  the  constitu- 


QUANTITY  OF  DILUTING  WATER  NEEDED  -377 

ents  of  the  salt  water  and  those  of  the  sewage,  serve  to  precipitate 
some  of  the  sewage  solids  and  to  form  offensive  sludge  banks. 
The  evidence  of  the  action  which  takes  place  in  the  absorption 
of  oxygen  from  the  atmosphere  by  salt  water  and  its  effect  on 
dissolved  sewage  solids  is  conflicting,  but  in  general  fresh  water 
is  a  better  diluting  medium  than  salt  water. 

Black  and  Phelps  have  made  valuable  studies  of  the  relative 
rates  of  absorption  of  oxygen  from  the  air  by  fresh  and  salt  water. 
The  results  of  their  experiments  are  published  in  a  Report  to  the 
Board  of  Estimate  and  Apportionment  of  N.  Y.  City,  made 
March  23,  191 1.1  Concerning  these  rates  they  conclude: 

Therefore  there  is  no  reason  to  believe  that  the 
reaeration  of  salt  water  follows  any  other  laws  than 
those  we  have  determined  mathematically  and  experi- 
mentally for  fresh  water.  In  the  absence  of  fuller  infor- 
mation on  the  effect  of  increased  viscosity  upon  the 
diffusion  coefficient,  it  can  only  be  stated  that  the  rate 
of  reaeration  of  salt  water  is  less  than  that  of  fresh  water, 
in  proportion  to  the  respective  solubilities  of  oxygen  in 
the  two  waters,  and  still  less,  but  to  an  unknown  extent, 
by  reason  of  the  greater  viscosity  and  consequent  small 
value  of  the  diffusion  coefficient. 

224.  Quantity  of  Diluting  Water  Needed. — In  a  large  majority 
of  the  problems  of  disposal  of  sewage  by  dilution  it  is  not  neces- 
sary to  add  sufficient  diluting  water  to  oxidize  completely  all 
organic  matter  present.  Ordinarily  it  is  sufficient  to  prevent 
putrefactive  conditions  until  the  flow  of  the  stream,  lake,  or 
tidal  current,  has  reached  some  large  body  of  diluting  water 
or  where  putrefaction  is  no  longer  a  nuisance.  It  is  never  desirable 
to  allow  the  oxygen  content  of  a  stream  to  be  exhausted  as  putres- 
cible  conditions  will  exist  locally  before  exhaustion  is  complete. 
The  exact  point  to  which  oxygen  can  be  reduced  in  safety  is  in 
some  dispute.  Black  and  Phelps  have  assumed  70  per  cent  of 
saturation  as  the  allowable  limit;  Fuller  has  placed  it  at  30 
per  cent;  Kinnicutt,  Winslow,  and  Pratt  have  placed  it  at  50 
per  cent.  Since  the  reaction  between  the  oxygen  and  the  organic 
matter  is  quantitative,  others  have  placed  the  limit  in  terms  of 
parts  per  million  of  oxygen.  Wisner,2  has  recommended  a  mini- 

1  Reprinted  in  Vol.  Ill  of  Contributions  from  the  Sanitary  Research 
Laboratory  of  Massachusetts  Institute  of  Technology. 

2  Formerly  Chief  Engineer  of  the  Sanitary  District  of  Chicago. 


378  DISPOSAL  BY  DILUTION 

mum  of  2.5  p.p.m.  as  the  limit  for  the  sustenance  of  fish  life, 
which  is  not  far  from  Fuller's  limit  for  hot-weather  conditions. 

Formulas  of  various  types  have  been  devised  to  express  the 
rate  of  absorption  of  oxygen  with  a  given  quantity  of  diluting 
water  which  is  mixed  with  a  given  quantity  and  quality  of  sewage. 
The  quantity  of  sewage  is  sometimes  expressed  in  terms  of  the 
tributary  population  or  in  other  ways.  Knowing  the  rate  at 
which  oxygen  is  exhausted  and  the  velocity  of  flow  of  the  stream, 
the  point  at  which  the  oxygen  will  be  reduced  to  the  limit  allowed 
is  easily  determined.  The  accuracy  of  none  of  these  formulas 
has  been  proven,  and  their  use,  without  an  understanding  of  the 
effect  of  local  conditions,  may  lead  to  error.  They  may  be  used 
as  a  check  on  the  bio-chemical  oxygen  demand  determinations, 
which  should  be  conclusive. 

The  following  formula,  based  on  the  work  of  Black  and 
Phelps,  is  a  guide  to  the  amount  of  sewage  which  can  be  added 
to  a  stream  without  causing  a  nuisance.  It  is: 

0' 

logo~ 


in  which  C  =per  cent  of  sewage  allowed  in  the  water; 

O'=per  cent  of  saturation  or  the  p.p.m.  of  oxygen  in 

the  mixture  at  the  time  of  dilution; 
0=per  cent  of  saturation  or  the  p.p.m.  of  oxygen  in 
the  stream  after  period  of  flow  to  point  beyond 
which  no  nuisance  can  be  expected; 
£=time  in  hours  required  for  the   stream  to  flow  to 

this  point; 

k  =  constant    determined    by    test    determinations    of 
the  factors  in  the  following  expression: 


k    - 

" 


in  which  O'i  =per  cent  of  saturation  or  the  p.p.m.  of  oxygen  in 

the  diluting  water  before  mixing  with  the  sewage; 

Oi=per  cent  of    saturation    or  the  p.p.m.  of  oxygen 

in  an  artificial  mixture  made  in  the  laboratory, 

after  t\  hours  of  incubation; 


QUANTITY  OF  DILUTING  WATER  NEEDED  379 

d  =  per  cent  of  sewage  in  the  mixture; 
ti  =  number  of  hours  of  incubation  of  the  mixture  of 
sewage   and    diluting  water    under    laboratory 
conditions. 

In  the  solution  of  these  formulas  it  is  desired  to  determine 
the  permissible  amount  of  sewage  to  discharge  into  a  given 
quantity  of  diluting  water.  This  value  is  expressed  by  C  in  the 
first  equation.  In  solving  this  equation: 

0'  is  determined  by  laboratory  tests  and  should  repre- 
sent the  conditions  to  be  expected  during 
various  seasons  of  the  year; 

0  is  determined  by  judgment.     It  may  be  30  per  cent 

or  50  per  cent  or  more  as  previously  explained; 

t  is  determined  by  float  tests  or  other  measurements 
of  the  stream  flow; 

k  is  determined  by  laboratory  tests  in  which  mix- 
tures of  various  strengths  are  incubated  for  vari- 
ous periods  of  time.  Different  values  of  k  will 
be  obtained  for  different  characteristics  of  the 
sewage;  but  for  the  same  sewage  the  value  of  k 
should  be  unchanged  for  different  periods  of 
incubation. 

Rideal  devised  the  formula: l 

XO=C(M-N)S 

in  which    -X"  =  flow  of  the  stream  expressed  in  second  feet; 

0  =  grams  of  free  oxygen  in  one  cubic  foot  of  water; 
S  =rate  of  sewage  discharge  in  second  feet; 
M=  grams  of  oxygen  required  to  consume  the  organic 
matter  in  one    cubic  foot  of    diluted   sewage  as 
determined    by   the    permanganate    test   with   4 
hours  boiling; 
N  =  grams  of    oxygen    available  in  the    nitrites  and 

nitrates  in  one  cubic  foot  of  diluted  sewage; 
C=  ratio  between  the  amount  of  oxygen  in  the  stream 
and  that  required  to  prevent  putrefaction.     Where 
C  is  equal  to  or  greater    than   one,  satisfactory 
conditions  have  been  attained. 

1  From  "  Sewage,"  by  Samuel  Rideal,  1900,  p.  16. 


380  DISPOSAL  BY  DILUTION 

In  using  this  formula  it  is  necessary  to  make  analyses  of  trial 
mixtures  of  sewage  and  water  until  the  correct  mixture  has  been 
found. 

Hazen's  formula  is:  1 


__ 

~S"  0' 

in  which  D  =  dilution  ratio; 

x  =  volume  of  water; 

S  =  volume  of  sewage; 

m=  result  of   the   oxygen   consumed   test   expressed  in 

p.  p.m.    after    5    minutes,   boiling  with    potassium 

permanganate  ; 
0  =  amount  of  dissolved  oxygen  in  the  diluting  water 

expressed  in  p.  p.m. 

For  comparison  with  Rideal's  formula  the  factor  of  7  should  be 
used  instead  of  4  to  allow  for  the  increased  time  of  boiling. 

Since  the  amount  of  oxygen  needed  is  dependent  on  the  amount 
of  organic  matter  in  the  sewage  rather  than  the  total  volume  of 
the  sewage,  and  since  the  amount  of  organic  matter  is  closely 
proportional  to  the  population,  the  amount  of  diluting  water  has 
sometimes  been  expressed  in  terms  of  the  population.  Bering's 
recommendation  for  the  quantity  of  diluting  water  necessary  for 
Chicago  sewage  was  3.3  cubic  feet  of  water  per  second  per 
thousand  population.  Experience  has  proven  this  to  be  too  small. 
Between  a  minimum  limit  of  2  second-feet  and  a  maximum  of  8 
second-feet  of  diluting  water  per  thousand  population  the  success 
of  dilution  is  uncertain.  Above  this  limit  success  is  practically 
assured  and  below  this  limit  failure  can  be  expected. 

Even  with  these  carefully  devised  formulas  and  empirical 
guides,  the  factors  of  reaeration,  dilution,  sedimentation,  tem- 
perature, etc.,  may  have  so  great  an  effect  as  to  vitiate  the  con- 
clusions. As  shown  in  Table  75  dilution  in  winter  is  far  more 
successful  than  in  summer.  The  lower  temperatures  so  reduce 
the  activity  of  the  putrefying  organisms  that  consumption  of 
oxygen  is  greatly  retarded. 

225.  Governmental  Control.  —  A  comprehensive  discussion 
of  the  legal  principles  governing  the  pollution  of  inland  waters 

1  See  Am.  Civil  Engineers'  Pocket  Book,  Second  Edition,  p.  982. 


PRELIMINARY  TREATMENT  381 

is  contained  in  "'A  Review  of  the  Laws  Forbidding  the  Pollution 
of  Inland  Waters,"  by  E.  B.  Goodell,  published  by  the  United 
States  Geological  Survey  in  1905,  as  Water  Supply  Paper  No.  152. 
The  disposal  of  sewage  by  dilution  is  subject  to  statutory 
limitations  in  many  states.  The  enforcement  of  these  laws  is 
usually  in  the  hands  of  the  state  board  of  health,  which  is  fre- 
quently given  discretionary  powers  to  recommend  and  some- 
times to  enforce  measures  for  the  abatement  of  an  actual  or 
potential  nuisance.  Such  recommendations  usually  take  the 
form  of  a  specification  of  certain  forms  of  treatment  preliminary 
to  disposal  by  dilution.  No  project  for  the  disposal  of  sewage 
by  dilution  should  be  consummated  until  the  local,  state, 
national,  and  in  the  case  of  boundary  waters,  international  laws 
have  been  complied  with.  The  attitude  of  the  courts  in  dif- 
ferent states  has  not  been  uniform.  Little  guidance  can  be  taken 
from  the  personal  feeling  of  the  persons  immediately  interested. 
The  opinion  of  the  riparian  owner  5  miles  down  stream  may  differ 
materially  from  the  popular  will  of  the  voters  of  a  city,  and  it  is 
likely  to  receive  a  more  favorable  hearing  from  the  court. 
Statutes  and  legal  precedents  are  the  safest  guides. 

226.  Preliminary  Treatment. — If  the  sewage  to  be  disposed 
of  by  dilution  contains  unsightly  floating  matter,  oil,  or  grease, 
no  amount  of  oxygen  in  the  diluting  water  will  prevent  a  nuisance 
to  sight,  or  the  formation  of  putrefying  sludge  banks.     Under 
such  conditions  it  will  be  necessary  to  introduce  screens  or  sedi- 
mentation basins,  or  both,  in  order  to  remove  the  floating  and  the 
settling  solids.     Biologic  tanks,  filtration,  or  other  methods  of 
treatment  may  be  necessary  for  the  removal  of  other  undesirable 
constituents. 

227.  Preliminary    Investigations.— Before    adopting    disposal 
of  sewage  by  dilution  without  preliminary  treatment,  or  before 
considering  the  proper  form  of  treatment  necessary  to  render 
disposal  by  dilution  successful,  a  study  should  be  made  of  the 
character  of  the  body  of  water  into  which  the  sewage  or  effluent 
is  to  be  discharged.     This  study  should  include:    measurements 
of  th?  quantity  of  water  available  at  all  seasons  of  the  year; 
analyses   of   the   diluting  water   to   determine   particularly  the 
available   dissolved  oxygen:    observations  of  the  velocity   and 
direction  of  currents,  and  the  effect  of  winds  thereon:    a  study 
of  the  effect  on  public  water  supplies,  bathing  beaches,  fish  life, 


382  DISPOSAL  BY  DILUTION 

etc.  Good  judgment,  aided  by  the  proper  interpretation  of 
such  information  should  lead  to  the  most  desirable  location  for 
the  sewer  outlet.  If  preliminary  treatment  is  found  to  be  neces- 
sary tests  should  be  made  to  determine  the  necessary  extent  and 
thoroughness  of  the  treatment. 


CHAPTER  XV 
SCREENING  AND   SEDIMENTATION 

228.  Purpose. — The  first  step  in  the  treatment  of  sewage 
is  usually  that  of  coarse  screening  in  order  to  remove  the  larger 
particles  of  floating  or  suspended  matter.  Screens  and  sedi- 
mentation basins  are  used  to  prevent  the  clogging  of  sewers, 
channels,  and  treatment  plants;  to  avoid  clogging  of  and  injuries 
to  machinery;  to  overcome  the  accumulation  of  putrefying 
sludge  banks;  to  minimize  the  absorption  of  oxygen  in  diluting 
water;  and  to  intercept  unsightly  floating  matter. 

By  the  plain  sedimentation  of  sewage  is  meant  the  removal 
of  suspended  matter  by  quiescent  subsidence  unaffected  by 
septic  action  or  the  addition  of  chemicals  or  other  precipitants. 
In  order  to  prevent  septic  action  plain  sedimentation  tanks  must 
be  cleaned  as  frequently  as  once  or  twice  a  week  in  warm  weather 
but  not  quite  so  often  in  cold  weather. 

Fine  screening  may  take  the  place  of  sedimentation  where 
insufficient  space  is  available  for  sedimentation  tanks,  and  it  is 
desired  to  remove  only  a  small  portion  of  the  suspended  matter. 
Recent  American  practice  has  tended  to  restrict  the  field  of  fine 
screening  to  treatment  requiring  less  than  10  per  cent  removal 
of  suspended  matter,  thus  eliminating  screens  from  the  field 
covered  by  plain  sedimentation  tanks.  The  practice  is  well 
expressed  by  Potter,  who  states: 1 

Where  a  high  degree  of  purification  is  sought,  the  use 
of  fine  screens  is  of  doubtful  value.  A  modern  settling 
tank  will  give  better  results  and  at  a  less  cost  for  a  given 
degree  of  purification.  A  settled  liquid  is  also  superior 
to  a  screened  liquid  for  subsequent  biological  treatment 
in  filters.  .  .  .  Again  the  storing  of  large  quantities  of 
screenings  must  necessarily  be  more  objectionable  than 
the  storing  of  the  digested  sludge  of  a  modern  settling 
tank. 

1  Trans.  Am.  Society  Civil  Engineers,  Vol.  58,  1907,  p.  988. 
383 


384 


SCREENING  AND  SEDIMENTATION 


229.  Types  of  Screens. — The  definitions  of  some  types  of 
screens  as  proposed  by  the  American  Public  Health  Association 
follow:  A  bar  screen  is  composed  of  parallel  bars  or  rods.  A 
mesh  screen  is  composed  of  a  fabric,  usually  wire.  A  grating 
consists  of  2  sets  of  parallel  bars  in  the  same  plane  in  sets  inter- 
secting at  right  angles.  A  band  screen  consists  of  an  endless 
perforated  band  or  belt  which  passes  over  upper  and  lower 
rollers.  A  perforated  plate  screen  is  made  of  an  endless  band 
of  perforated  plates  similar  to  a  band  screen.  A  wing  screen 
has  radial  vanes  uniformly  spaced  which  rotate  on  a  horizontal 
axis.  A  disc  screen  consists  of  a  circular  perforated  disc  with 
or  without  a  central  truncated  cone  of  similar  material  mounted 


f.-Band  Screen.        2.- Wing  Screen. 


3.- Shove! -Vane 
Screen. 


4.- Drum  Screen. 


5-Riensch-Wurl 
Screen. 


FIG.  150. — Types  of  Moving  Screens. 

Trans.  Am.  Society  Civil  Engineers,  Vol.  78,  1915,  p.  893. 

in  the  center.  The  Reinsch  Wurl  screen  is  the  best  known  type 
of  disc  screen.  A  cage  screen 1  consists  of  a  rectangular  box 
made  up  of  parallel  bars  with  the  upstream  side  of  the  box  or  cage 
omitted.  Allen2  gives  the  following  definitions:  A  drum 
screen  is  a  cylinder  or  cone  of  perforated  plates  or  wire  mesh 
which  rotates  on  a  horizontal  axis.  A  shovel  vane  screen  is 
similar  to  a  wing  screen  with  semicircular  wings  and  a  different 
method  of  removing  the  screenings.  Examples  of  a  band  screen, 
a  wing  screen,  a  shovel  vane  screen,  a  drum  screen  and  a  disc 

1  Not  defined  by  the  American  Public  Health  Association. 

2  Trans.  Am.  Society  Civil  Engineers,  Vol.  78,  1915,  p.  892. 


TYPES  OF  SCREENS  385 

screen  are  shown  in  Fig.  150.      A  bar  screen  is  shown  in  Fig. 
151  and  a  cage  screen  is  shown  in  Fig.  152. 


Direction 
of  Flow 


1 

I 

1--- 

-.:.:.:.. 

1 

1 

\ 

Side  View. 


Front  View 
Looking  down  Stream. 


FIG.  151. — Sketch  of  a  Bar  Screen. 


Screens  can  be  classed  as  fixed,  movable,  or  moving.  Fixed 
screens  are  permanently  set  in  position  and  must  be  cleaned 
by  rakes  or  teeth  that  are  pulled  between  the  bars.  Movable 
screens  are  stationary  when  in 
operation,  but  are  lifted  from 
the  sewage  for  the  purpose  of 
cleaning.  Moving  screens  are 
in  continuous  motion  when  in 
operation  and  are  cleaned  while 
in  motion.  Fixed  bar  screens 
may  be  set  either  vertical,  in- 
clined, or  horizontal. 

Movable  screens  with  a 
cage  or  box  at  the  bottom 
are  sometimes  used.  Ihe  box 
should  be  of  solid  material 
to  prevent  the  forcing  of 


FIG.  152.— Sketch  of  a  Cage  Screen. 


screenings    through     it     when 

the  screen  is   being   raised   for 

cleaning.      A   mesh    screen   should   be   used  only  under  special 

circumstances    because    of   the    difficulty    in    cleaning.     Screens 

which  must  be  raised  from  the  sewage  for  cleaning  should  be 


386  SCREENING  AND  SEDIMENTATION 

arranged  in  pairs  in  order  that  one  may  be  working  when  the 
other  is  being  cleaned.  Movable  screens  are  undesirable  for 
small  plants  because  the  labor  involved  in  raising  and  lowering 
is  greater  than  in  cleaning  with  a  rake  and  the  screens  are  more 
likely  to  be  neglected.  In  a  large  plant  rakes  operated  by  hand 
are  too  small  for  cleaning  the  screens.  A  fixed  screen  is  sometimes 
used  with  moving  teeth  fastened  to  endless  chains.  The  teeth 
pass  between  the  parallel  bars  and  comb  out  the  screenings.  If 
the  screen  chamber  in  a  small  plant  is  too  deep  for  accessibility 
a  movable  cage  or  box  screen  may  be  desirable. 

Moving  screens  are  generally  of  fine  mesh  or  perforated  plates. 
They  are  kept  moving  in  order  to  allow  continuous  cleaning. 
They  are  cleaned  by  brushes  or  by  jets  of  air,  water,  or  steam. 

230.  Sizes  of  Openings. — The  area  or  size  of  the  opening  of 
a  screen  is  dependent  upon  the  character  of  the  sewage  to  be 
treated  and  upon  the  object  to  be  attained. 

Large  screens,  with  openings  between  1^  inches  and  6  inches 
are  used  to  protect  centrifugal  pumps,  tanks,  automatic  dosing 
devices,  conduits,  and  gate  valves  from  large  objects  such  as 
pieces  of  timber,  dead  animals,  etc.,  which  are  found  in  sewage. 
The  quantity  of  material  removed  is  variable,  and  is  usually 
small. 

Medium-size  screens  with  openings  from  J  inch  to  1J  inches 
are  used  to  prepare  sewage  for  passage  through  reciprocating 
pumps,  complex  dosing  apparatus,  contact  beds,  and  sand  filters. 
The  amount  of  material  removed  varies  from  0.5  to  10  cubic 
feet  per  million  gallons  of  sewage  treated,  dependent  on  the 
character  of  the  sewage  and  the  size  of  the  screen.  Screenings 
before  drying  contain  75  to  90  per  cent  moisture  and  weigh 
40  to  50  pounds  per  cubic  foot.  At  times  the  amount  removed 
may  vary  widely  from  the  limits  stated.  Schaetzle  and  Davis 1 
state: 

Screenings  differ  greatly  both  in  amount  and  character. 
.  .  .  The  amount  varies  with  the  days  of  the  week  as 
well  as  during  the  course  of  the  day.  It  reaches  its 
maximum  about  noon  or  shortly  before  and  commences 
to  disappear  about  midnight,  reaching  a  mimimum  about 
4  or  5  A.M.  The  material  is  almost  wholly  organic  and 

1  Removal  of  Suspended  Matter  by  Sewage  Screens,  Cornell  Civil  En- 
gineer, 1914.  Abstracted  in  Engineering  and  Contracting,  Vol.  41,  1914, 
p.  451. 


SIZES  OF  OPENINGS  387 

consists  of  scraps  of  vegetables  or  fruit,  cloth,  hair,  wood, 
paper  and  lumps  of  fecal  matter.  The  amount  varies  so 
widely  that  it  is  impossible  to  state  just  what  to  expect 
any  definite  size  screen  to  remove.  The  amount  of 
water  contained  is  small  compared  with  that  in  the 
sludge  in  sedimentation  basins  and  amounts  to  from  70 
per  cent  to  80  per  cent.  On  account  of  its  organic  origin 
it  is  highly  putrescible. 

Medium-size  screens  are  sometimes  placed  close  together  with 
the  bars  of  the.  one  opposite  the  openings  in  the  other,  thus 
approaching  a  fine  screen. 

Fine  screens  vary  in  size  of  opening  from  J  inch  to  50  open- 
ings per  linear  inch  or  2,500  per  square  inch.  They  are  used  for 
removing  solids  preparatory  to  disposal  by  dilution,  to  protect 
sprinkling  filters,  complex  dosing  apparatus,  sand  filters,  sewage 
farms,  and  to  prevent  the  formation  of  scum  in  subsequent 
tank  treatment.  In  general,  fine  screens  will  remove  from  0.1 
to  1  cubic  yard  of  wet  material  per  million  gallons  of  sewage 
treated.  The  wet  screenings  will  contain  about  75  per  cent 
moisture  and  will  weigh  about  60  pounds  per  cubic  foot.  The 
dry  weight  of  the  screenings  will  therefore  be  about  10  to  400 
pounds  per  million  gallons  of  sewage  treated.  The  effect  of  the 
removal  of  this  amount  of  material  is  usually  not  detectable  by 
methods  of  chemical  analysis,  the  amount  of  suspended  matter 
before  and  after  screening  being  found  unchanged. 

In  his  conclusions  on  the  discussion  of  the  results  to  be 
expected  from  fine  screens,  Allen  states:1 

With  openings  not  more  than  0.1  inch  in  size,  fine 
screening  should  remove  at  least  30  per  cent  of  the  sus- 
pended solids  and  20  per  cent  of  the  suspended  organic 
solids  from  ordinary  domestic  sewage,  or  0.1  cubic  yard 
of  screenings,  containing  75  per  cent  water  per  thousand 
population  daily, 

The  effect  of  the  use  of  different  size  openings  under  the  same 
conditions  is  shown  in  Fig.  153.2  Some  data  covering  the  amount 
of  material  removed  by  screening  are  given  in  Table  76.  More 

1  "  The  Clarification  of  Sewage  by  Fine  Screens,"  Trans.  Am.  Society 
Civil  Engineers,  Vol.  78,  1915,  p.  1000. 

2  Langdon  Pearse,  Trans.  Am.   Society  Civil  Engineers,  Vol.  78,    1915, 
p.  1000. 


388 


SCREENING  AND  SEDIMENTATION 


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DESIGN  OF  FIXED  AND   MOVABLE  SCREENS 


389 


extensive  data  are  given  in  Volume  III  of  "American  Sewerage 
Practice"  by  Metcalf  and  Eddy. 

231.  Design  of  Fixed  and  Movable  Screens. — The  determina- 
tion of  the  size  of  the  opening  is  the  first  step  in  the  design  of 
a  sewage  screen.  This  is  followed  by  the  computation  of  the  net 
area  of  openings  in  the  screen.  The  final  steps  are  the  deter- 


0.20 


Removal  by  Screening. 
O  Individual  Results  on  Stockyards  Sewage 
+  Average  Results  on  Thirty-n 


No. 4  Screen 


No.6  Screen 


-No.  tO  Screen 


No.  16  Screen 
-No.  20 Screen 
-No.  24  Screen 
-  No.  50  Screen 
-No. 40 Screen 


400        800         1200        1600       2000      2400       2800 
Dry  Screenings  in  Pounds  per  Million  Gallons. 

FIG.  153. — Screenings  Collected  on  Different  Sized  Opening. 

1921  Report  on  Industrial  Wastes  Disposal,  Union  Stock  Yards  District,  Chicago,  Illinois, 
to  the  Sanitary  District  of  Chicago. 

mination  of  the  overall  dimensions  of  the  screen;  the  size  of  the 
bar,  wire,  or  support;  and  the  dimensions  of  the  screen  chamber. 
The  net  area  of  openings  is  fixed  by  the  permissible  velocity  of 
flow  through  the  screen  and  the  quantity  of  sewage  to  be  treated. 
In  determining  the  velocity  of  flow  the  general  principle  should 
be  followed  that  the  velocity  should  not  be  reduced  sufficiently 


390  SCREENING  AND  SEDIMENTATION 

to  allow  sedimentation  in  the  screen  chamber.  The  velocity 
of  grit  bearing  sewage  in  passing  through  coarse  screens  should 
not  be  reduced  below  2  or  3  feet  per  second.  If  the  sewage  con- 
tains no  grit,  or  the  screen  is  placed  below  a  grit  chamber  the 
velocity  through  a  medium  or  fine  screen  should  be  from  J  to  1J 
feet  per  minute.  The  velocity  through  the  screen  in  a  direction 
normal  to  the  plane  of  the  screen  can  be  reduced  without  reducing 
the  horizontal  velocity  of  the  sewage  by  placing  the  screen  in  a 
sloping  position. 

The  final  steps  are  the  design  of  the  screen  bar  and  the  deter- 
mination of  the  dimensions  of  the  screen  and  of  the  screen  chamber. 
The  size  of  the  bar  in  -a  bar  screen,  or  as  a  support  to  a  wire  mesh, 
is  dependent  on  the  unsupported  length  of  the  bar.  The  stresses 
in  the  bars  are  the  results  of  impact  and  bending,  caused  by  clean- 
ing, and  of  the  load  due  to  the  backing  up  of  the  sewage  when  the 
screen  is  clogged.  Allowance  should  be  made  for  a  head  of 
2  or  3  feet  of  sewage  against  the  screen.  A  generous  allowance 
should  be  made  in  addition  for  the  indeterminate  stresses  due  to 
cleaning.  The  screen  should  be  supported  only  at  the  top  and 
bottom,  as  intermediate  supports  in  a  bar  screen  are  undesirable 
unless  they  are  so  arranged  as  not  to  interfere  with  the  teeth 
of  the  cleaning  devices. 

Fixed  screens  should  be  placed  at  an  angle  between  30°  and 
60°  with  the  horizontal,  with  the  direction  of  slope  such  that  the 
screenings  are  caught  on  the  upper  portion  of  the  screen.  A 
small  slope  is  desirable  in  order  to  obtain  a  low  velocity  through 
the  screen.  The  slope  is  limited  since  the  smaller  the  slope  the 
longer  the  bars  of  the  screen  and  the  greater  the  difficulty  of  hand 
cleaning.  Small  slopes  will  tend  to  make  the  screens  self  cleaning. 
As  the  screen  clogs,  the  increasing  head  of  sewage  will  push  the 
accumulated  screenings  up  the  screen.  The  use  of  flat  screens 
in  a  vertical  position  is  not  desirable  because  of  the  difficulty  of 
cleaning  and  the  accumulation  of  material  at  inaccessible  points. 
If  a  flat  screen  is  placed  in  a  horizontal  position  with  the  flow  of 
sewage  downward  difficulties  are  encountered  in  cleaning  and 
solid  matter  is  forced  through  the  screen  as  clogging  increases. 
An  upward  flow  through  a  horizontal  screen  is  undesirable  as  the 
material  is  caught  in  a  position  inaccessible  for  cleaning.  Movable 
screens  are  more  easily  handled  when  placed  in  a  vertical  position. 

In  the  construction  of  small  screens,  round  bars  are  sometimes 


THEORY  OF  SEDIMENTATION  391 

used  where  the  unsupported  length  of  the  bar  is  less  than  3  or 
4  feet.  They  are  not  recommended,  however,  as  the  efficient 
area  and  the  amount  of  material  removed  by  the  screen  are 
diminished.  Bars  which  produce  openings  with  the  larger 
end  upstream  are  undesirable,  as  particles  become  wedged  in 
the  screen,  and  are  either  forced  through  or  become  difficult 
to  remove.1  Rectangular  bars  are  easily  obtained  and  give 
satisfactory  service  except  where  they  are  of  insufficient  strength 
laterally.  For  greater  lateral  thickness  a  pear-shaped  bar  is 
sometimes  used,  with  the  thicker  side  upstream.  Fine  mesh 
screens  or  perforated  plates  are  supported  on  grids  or  parallel 
bars  of  stronger  material  designed  to  take  up  the  heavy  stresses 
on  the  screen. 

The  dimensions  of  the  bar  may  be  selected  arbitrarily.  The 
length  and  width  of  the  screen  are  fixed  to  give  desirable  dimen- 
sions to  the  screen  chamber  and  to  give  the  necessary  net  opening 
in  the  screen.  The  width  of  the  screen  chamber  and  the  screen 
should  be  the  same.  The  screen  chamber  should  be  sufficiently 
long  to  prevent  swirling  and  eddying  around  the  screen.  If  the 
dimensions  thus  fixed  permit  an  undesirable  velocity  in  the  screen 
chamber  they  should  be  changed.  A  sufficient  length  of  screen 
should  be  allowed  to  project  above  the  sewage  for  the  accumula- 
of  screenings.  The  bars  may  be  carried  up  and  bent  over  at 
the  top  as  shown  in  Fig.  151  to  simplify  the  removal  of  screenings. 

Coarse  screens  are  usually  placed  above  all  other  portions 
of  a  treatment  plant.  They  may  be  followed  by  grit  chambers 
or  finer  screens.  Coarse  screens  are  occasionally  placed  as  a 
protection  above  medium  or  fine  screens.  In  sewage  containing 
grit  the  smaller  screens  are  sometimes  placed  below  the  grit  cham- 
ber. It  is  desirable  to  provide  some  means  of  diverting  the  sewage 
from  a  screen  chamber  to  allow  of  repairs  to  the  screen  and  the 
cleaning  of  the  chamber.  Screen  chambers  are  sometimes 
designed  in  duplicate  to  allow  for  the  cleaning  of  one  while  the 
other  is  operating. 

PLAIN  SEDIMENTATION 

232.  Theory  of  Sedimentation. — Sedimentation  takes  place  in 
sewage    because   some    particles   of   suspended   matter   have   a 
greater  specific  gravity  than  that  of  water.     All  particles   do 
1  See  article  by  Henry  Ryon  in  Cornell  Civil  Engineer,  1910. 


392  SCREENING  AND  SEDIMENTATION 

not  settle  at  the  same  rate.  Since  the  weights  of  particles  vary 
as  the  cubes  of  their  diameters,  whereas  the  surface  areas  (upon 
which  the  action  of  the  water  takes  place)  vary  only  as  the 
squares  of  the  diameters,  the  amount  of  the  skin  friction  on 
small  particles  is  proportionally  greater  than  that  on  large 
particles,  because  of  the  relatively  greater  surface  area  compared 
to  their  weight.  As  a  result  the  smaller  particles  settle  more 
slowly.  The  velocity  of  sedimentation  of  large  particles  has 
been  found  to  vary  about  as  the  diameter  and  of  small  particles 
as  the  square  root  of  the  diameter.  The  change  takes  place  at 
a  size  of  about  0.01  mm. 

Sedimentation  is  accomplished  by  so  retarding  the  velocity 
of  flow  of  a  liquid  that  the  settling  particles  will  be  given  the 
opportunity  to  settle  out.  The  slowing  down  of  the  velocity 
is  accomplished  by  passing  the  sewage  through  a  chamber  of 
greater  cross  sectional  area  than  the  conduit  from  which  it  came. 
The  time  that  the  sewage  is  in  this  chamber  is  called  the  period 
of  retention.  Although  the  shape  of  a  basin,  the  arrangement 
of  the  baffles  and  other  details  have  a  marked  effect  on  the 
results  of  sedimentation,  the  controlling  factors  are  the  period 
of  retention  and  the  velocity  of  flow.  Another  factor  affecting 
the  efficiency  of  the  process  is  the  quality  of  the  sewage.  Usually 
the  greater  the  amount  of  sediment  in  the  sewage  the  greater  the 
per  cent  of  suspended  matter  removed.  A  method  for  the 
determination  of  the  proper  period  of  sedimentation  has  been 
developed  by  Hazen  in  Transactions  of  the  American  Society  of 
Civil  Engineers,  Volume  53,  1904,  page  45.  The  results  of 
his  studies  are  summarized  in  Fig.  154  which  shows  the  per  cent 
of  sediment  remaining  in  a  treated  water  after  a  certain  period 
of  retention.  This  period  of  retention  is  expressed  in  terms  of 
the  hydraulic  coefficient 1  of  the  smallest  size  particle  to  be 
removed.  Table  77  shows  the  hydraulic  coefficients  of  various 
particles.  In  Fig.  154  a  represents  the  period  of  retention  and 
t  the  time  that  it  would  take  a  particle  to  fall  to  the  bottom  of 
the  basin.  The  different  lines  of  the  diagram  represent  the 
results  to  be  expected  by  various  arrangements  of  settling  basins. 
The  meaning  of  these  lines  is  given  in  Table  78. 

1  The  hydraulic  coefficient  is  defined  as  the  rate  of  settling  in  mm.  per 
second. 


THEORY  OF  SEDIMENTATION 


393 


TABLE  77 
HYDRAULIC  VALUES  OF  SETTLING  PARTICLES  IN  MILLIMETERS  PER  SECOND 


Diameter 
in 
mm. 

Hy- 
draulic 
Value 

Diameter 
in 
mm. 

Hy- 

draulic 
Value 

Diameter 
in 
mm. 

Hy- 
draulic 
Value 

eter 
in 

Hydraulic 
Value 

mm. 

1.00 

100 

0.15 

15 

0.02 

0.62 

0.003 

0.0138 

0.80 

83 

0.10 

8 

0.015 

0.35 

0.002 

0.0062 

0.60 

63 

0.08 

6 

0.010 

0.154 

0.0015 

0.0035 

0.50 

53 

0.06 

3.8 

0.008 

0.098 

0.001 

0.00154 

0.40 

42 

0.05 

2.9 

0.006 

0.055 

0.0001 

0.0000154 

0.30 

32 

0.04 

2.1 

0.005 

0.0385 

0.20 

21            0.03 

-     H 

1.3 

0.004 

0.0247 

An  example  will  be  given  to  illustrate  the  method  of  using  the 
diagram  and  tables  to  determine  the  size  of  a  sedimentation 
basin  to  perform  certain  required  work. 

Let  it  be  required  to  determine  the  period  of  retention 
in  a  continuously  operated  sedimentation  basin  with  good 
baffling,  corresponding  to  two  properly  baffled  sedimentation 
basins  in  series.  The  basins  are  to  remove  60  per  cent  of 
the  finest  particles  which  are  to  have  a  size  of  0.01  mm. 
The  quantity  to  be  treated  daily  is  3,000,000  gallons. 

1st.  Entering  Table  77,  we  find  that  the  hydraulic 
value  of  the  finest  particles  is  0.154  mm.  per  second. 

2d.  Since  we  wish  to  remove  60  per  cent  of  the  finest 
particles,  40  per  cent  will  remain.  Since  Fig.  154  shows 
the  per  cent  remaining  after  the  time  a/t  we  enter  Fig. 
154  at  40  per  cent  on  the  ordinates  and  run  horizontally 
until  we  encounter  Line  4  corresponding  to  good  baffling 
in  Table  78.  We  then  run  down  vertically  from  this 
intersection  and  find  that  the  ratio  of  a/t  is  1.0. 

Then  a  equals  t,  which  means  that  the  period  of  reten- 
tion should  equal  the  time  that  it  takes  a  particle  0.01  mm. 
in  diameter  to  drop  from  the  top  to  the  bottom  of  the 
basin.  Since  this  depends  on  the  depth  of  the  basin  it 
is  necessary  to  determine  the  depth  before  the  other 
dimensions  of  the  basin  can  be  fixed. 

Although  this  method  is  seldom  used  in  practice  for  the  final 
design  of  a  sedimentation  basin,  it  is  a  guide  to  judgment  and 
can  be  used  to  supplement  the  data  obtained  from  tests, 


394 


SCREENING  AND  SEDIMENTATION 


0  0.5  1.0  1.5  2.0  2.5  3.0  3.5 

•jr,  or  Time  of  Settling, in  Terms  of  Time  Required  for  One  ParticletoSettlefromToptoBottom. 

FIG.  154.— Hazen's  Diagram,  Showing  the  Relation  between  the  Time  of  Set- 
tling and  the  Period  of  Retention  in  Various  Types  of  Sedimentation 
Basins. 

Trans.  Am.  Society  Civil  Engineers,  Vol.  53,  1904,  p.  45. 

TABLE  78 

COMPARISON  OF  DIFFERENT  ARRANGEMENTS  OF  SETTLING  BASINS 
(From  Hazen) 


Description  of  Basins 

Line 
in 
Fig. 
154 

Values  of  a/t. 

Per  Cent  of  Matter 
Removed 

50 

74 

87.5 

Theoretical  maximum.     Cannot  be  reached.  .  .  . 
Surface  skimming.     Rockner  Roth  system  
Intermittent  basins,  reckoned  on  time  of  service 
only 

A 
B 

C 
D 
16 
8 
4 
2 
1.5 
E 
1 

0.50 
0.54 

0.63 
0.69 
0.71 
0.73 
0.76 
0.82 
0.90 
1.26 
1.00 

0.75 

0.98 

1.26 
1.38 
1.45 
1.62 
1.66 
2.00 
2.34 
2.50 
3.00 

0.875 
1.37 

1.89 
2.08 
2.23 
2.37 
2.75 
3.70 
4.50 
3.80 
7.00 

Continuous  basin.     Theoretical  limit  

Close  approximation  to  the  above    

Very  well  baffled  basin 

Good  baffling  

Two  basins  tandem                       

One  long  basin  well  controlled 

Intermittent  basin  in  service  half  time  

One  basin   continuous                            • 

TYPES  OF  SEDIMENTATION  BASINS  395 

The  design  of  sedimentation  basins  should  be  based  on 
experimental  observations  made  upon  the  quantity  of  sediment 
removed  at  certain  rates  of  flow  and  periods  of  retention  in 
different  types  of  basins.  Hazen's  mathematical  analysis  is  service- 
able in  making  preliminary  estimates  and  in  checking  the  results. 
The  shape  of  the  tank,  period  of  retention  and  rate  of  flow  pro- 
ducing the  most  desirable  results  should  be  duplicated  with  the 
expectation  of  obtaining  similar  results  or  results  but  slightly 
modified  from  those  obtained  in  the  tests.  This  is  the  most 
satisfactory  method  of  determining  the  proper  period  of  retention. 

233.  Types  of  Sedimentation  Basins. — A  sedimentation  basin 
is  a  tank  for  the  removal  of  suspended  matter  either  by  quiescent 
settlement  or  by  continuous  flow  at  such  a  velocity  and  time  of 
retention  as  to  allow  deposition  of  suspended  matter.1  The 
difference  between  sedimentation  tanks  and  other  forms  of  tank 
treatment  is  that  no  chemical  or  biological  action  is  depended 
on  for  the  successful  operation  of  the  tank.  Sedimentation 
tanks  may  be  divided  into  two  classes,  grit  chambers  and  plain 
sedimentation  basins. 

A  grit  chamber  is  a  chamber  or  enlarged  channel  in  which  the 
velocity  of  flow  is  so  controlled  that  only  heavy  solids,  such  as 
grit  and  sand,  are  deposited  while  the  lighter  organic  solids  are 
carried  forward  in  suspension.  If  the  velocity  of  flow  is  more 
than  about  one  foot  per  second,  the  tank  is  a  grit  chamber  and 
below  this  velocity  it  is  a  plain  sedimentation  basin. 

There  are  sixth  general  types  of  plain  sedimentation 
basins: 

1st.  Rectangular  flat-bottom  tanks  operated  on  the 
continuous-flow  principle. 

2nd.  Rectangular  flat-bottom  tanks  operated  on  the 
fill  and  draw  principle. 

3rd.  Rectangular  or  circular  hopper-bottom  tanks 
operated  on  the  continuous-flow  principle,  with  hori- 
zontal flow. 

4th.  Rectangular  or  circular  hopper-bottom  tanks 
operated  on  the  fill  and  draw  principle,  with  horizontal 
flow. 

5th.  Rectangular  or  circular  hopper-bottom  tanks 
operated  on  the  continuous-flow  principle  with  vertical  flow. 

6th.  Circular  hopper-bottom  tanks  operated  on  the 
continuous-flow  principle  with  radial  flow. 

1  Definition  suggested  by  the  American  Public  Health  Association. 


396 


SCREENING  AND  SEDIMENTATION 


TABLE  79 

CRITICAL  VELOCITIES  FOR  THE  TRANSPORTATION  OP  DEBRIS 
Sedimentation  will  not  Occur  at  Higher  Velocities 


Diameter 
of  Particle 

Critical  Velocity,  Feet  per  Second. 

Size  of  Screen 
or  Number  of 

Specific  Gravity 

in 

Millimeters 

1.5 

2.0 

3.0 

5.0 

Meshes  per  Inch 

0.010 

0.13 

0.20 

0.22 

0.28 

0.050 

0.23 

0.34 

0.39 

0.50 

More  than  200 

0.100 

0.30 

0.42 

0.50 

0.65 

More  than  150 

0.500 

0.55 

0.73 

0.91 

1.15 

More  than    28 

1.0 

0.71 

0.92 

1.18 

1.50 

More  than    14 

1.25 

0.77 

1.00 

1.30 

1.60 

2.0 

0.92 

1.20 

1.50 

1.90 

More  than    10 

5.0 

1.30 

1.70 

2.20 

2.60 

More  than      4 

10 

1.70 

2.20 

2.8 

3.4 

Diameter  in  Millimeters  for  a  Velocity  of  1  Foot  per  Second 


2.5 

1.25 

0.65 

0.32 

234.  Limiting  Velocities. — Sand,  clay,  bits  of  metal  and  other 
particles  of  mineral  matter  will  commence  to  deposit  in  appreci- 
able quantities  when  the  velocity  of  flow  falls  below  3  feet  per 
second.  The  amount  deposited  will  increase  as  the  velocity 
decreases.  In  Table  79  are  given  the  approximate  horizontal 
velocities  at  which  certain  size  particles  of  mineral  matter  will 
deposit.  At  a  velocity  of  about  one  foot  per  second  organic 
matter  will  commence  to  deposit.  It  will  be  noticed  by  inter- 
polation in  Table  79,1  that  particles  with  the  same  specific 
gravity  as  sand  (2.6),  larger  than  one  mm.  in  diameter  will 
deposit  at  a  velocity  of  about  one  foot  per  second  or  less,  and 
that  smaller  and  lighter  particles  will  not  deposit  at  velocity 
of  one  foot  per  second  or  greater.  It  will  also  be  noticed  that  a 

1  Computed  from  formula  by  Gilbert  in  "  Transportation  of  Debris  by 
Running  Water,"  U.  S.  Geological  Survey,  Professional  Paper  No.  86,  1914. 
1.28  (velocity)2'7 


Diameter  in  mm.  = 


Sp.gv.-l 


QUANTITY  AND  CHARACTER  OF  GRIT 


397 


velocity  of  one  foot  per  minute  is  sufficiently  slow  to  permit  the 
deposit  of  the  smallest  and  lightest  particles.  For  this  reason 
velocities  of  1  or  2  or  even  3  feet  per  second  have  been  adopted 
as  the  velocities  in  grit  chambers  and  velocities  less  than  1  foot 
per  minute  in  plain  sedimentation  basins. 

235.  Quantity  and  Character  of  Grit. — The  amount  of 
material  deposited  in  grit  chambers  varies  approximately  between 
0.10  and  0.50  cubic  yard  per  million  gallons.  It  is  to  be  noted 
that  grit  chambers  are  used  only  for  combined  and  storm  sewage 
and  for  certain  industrial  wastes.  They  are  unnecessary  for 
ordinary  domestic  sewage.  The  material  deposited  in  grit  cham- 
bers operating  with  a  velocity  greater  than  one  foot  per  second 
is  nonputrescible,  inorganic,  and  inoffensive.  It  can  be  used  for 
filling,  for  making  paths  and  roadways,  or  as  a  filtering  material 
for  sludge  drying  beds.  An  analysis  of  a  typical  grit  chamber 
sludge  is  shown  in  Table  80. 

TABLE  80 
ANALYSIS  OF  GRIT  CHAMBER  SLUDGE 


Velocity 
Feet  per 
Second 

Specific 
Gravity 

Per  Cent 
Moisture 

Calculated  to  Dry  Weight.  Per  Cent 

Nitrogen 

Fixed  Matter 

Miscellaneous 

1.0 

1.5 

45 

20 

78 

2 

236.  Dimensions  of  Grit  Chambers. — The  quantity  of  sewage 
to  be  treated  and  the  amount  and  character  of  the  settling  solids 
which  it  contains  should  be  determined  by  measurement  and 
analysis,  and  the  amount  of  settling  solids  to  be  removed  should  be 
determined  by  a  study  of  the  desired  conditions  of  disposal,  in 
order  that  a  grit  chamber  that  will  accomplish  the  desired  results 
may  be  designed.  The  period  of  retention  and  the  velocity  of 
flow  are  the  controlling  features  in  the  successful  operation  of 
any  grit  chamber.  These  should  be  determined  by  experiment 
or  as  the  result  of  experience.  Where  neither  are  available, 
Hazen's  method  can  be  followed  or  a  decision  made  based  on  a 
study  of  other  grit  chambers.  In  general,  the  period  of  retention 


398  SCREENING  AND  SEDIMENTATION 

in  grit  chambers  is  from  30  to  90  seconds,  and  the  velocity  of  flow 
is  about  one  foot  per  second. 

After  having  determined  the  quantity  of  sewage  to  be  treated, 
the  quantity  of  grit  to  be  stored  between  cleanings,  the  period 
of  retention,  the  arrangement  of  the  chambers,  and  the  velocity 
of  flow  to  be  used,  the  overall  dimensions  of  the  chambers  are 
computed.  The  capacity  of  the  chamber  is  fixed  as  the  sum  of 
the  quantity  of  sewage  to  be  treated  during  the  period  of  reten- 
tion and  the  required  storage  capacity  for  grit  accumulated 
between  cleanings.  The  length  of  the  chamber  is  fixed  as  the 
product  of  the  velocity  of  flow  and  the  period  of  retention.  The 
cross-sectional  area  of  the  portion  of  the  chamber  devoted  to 
sedimentation  is  fixed  as  the  quotient  of  the  quantity  of  flow  of 
sewage  per  unit  time  and  the  velocity  of  flow.  Only  the  relation 
between  the  width  and  depth  of  the  portion  devoted  to  sedi- 
mentation and  the  portion  devoted  to  the  storage  of  grit  remain 
to  be  determined.  These  should  be  so  designed  as  to  give  the 
greatest  economy  of  construction  commensurate  with  the  required 
results.  They  will  be  affected  by  the  local  conditions  such  as 
topography,  available  space,  difficulties  of  excavation,  etc.  Com- 
mon depths  in  use  lie  between  8  and  12  feet,  although  wide  varia- 
tions can  be  found.  A  study  of  the  proportions  of  existing 
grit  chambers  will  be  of  assistance  in  the  design  of  other 
basins. 

237.  Existing  Grit  Chambers. — The  details  of  some  typical 
grit  chambers  are  shown  in  Figs.  155  and  156.  The  grit  chamber 
at  the  foot  of  58th  Street,  in  Cleveland,  Ohio,  is  shown  in  Fig. 
155.  The  special  feature  of  this  structure  is  the  shape  of  the 
sedimentation  basin,  the  bottom  of  which  is  formed  by  sloping 
steel  plates  forming  a  6-inch  longitudinal  slot  above  the  grit 
storage  chamber.  Flows  between  8,000,000  and  16,000,000 
gallons  per  day  are  controlled  by  the  outlet  weir  so  that  the 
velocity  of  flow  remains  at  one  foot  per  second.  This  is  accom- 
plished by  increasing  the  depth  of  flow  in  the  same  ratio  as  the 
increase  in  the  rate  of  flow.  The  bottoms  of  the  two  chambers 
differ,  one  having  a  special  hopper  for  grit  and  the  other  a  flat 
bottom.  This  is  due  to  the  method  of  cleaning  the  chambers, 
it  being  necessary  in  the  one  with  a  flat  bottom  to  shut  off  the 
flow  when  removing  the  grit  while  in  the  one  with  the  hopper 
bottom  it  is  hoped  to  remove  the  grit  by  the  use  of  sand  ejectors 


EXISTING  GRIT  CHAMBERS 

K__,_C_"v so'-O" •*!  .A 


399 


^L7^mfs'-7-^---- "     "  '^7-i3i&7-i3&'- 

$***</,    '•               Section  A-A. 
SO'-O" -* 


6"C.f.Pipe 


w^^T/i  •  Ti^;;.- 
Section  C-Ci> 


econ- 
FIG.  155. — Grit  Chamber  at  Cleveland,  Ohio. 

Eng.  Record,  Vol.  73,  1916,  p.  409. 


•  n*  ,-Welr      .•tflt'Steel  CiSOIbs. 

~°~ """^^'      P*"Sl6cksr,Screen  '    2l6',.ncf 

•*K&~Ls.H lfluan VJl         II          f&g 


.,     r4'^7//tf 

LI-?WS*-J_I       Chamber" 

,6-MKp.     -|r 

/          Open  Join  fs      :;;[[ 


Screen 


Se/ectrt 
DryFi'lling 
in  Layers^ 


Plan. 

Wj/'r  Penstocks 
By-pan . 


. Valve 


^ft  tl  rT  r*»  n 


jj  j       To  Effluent  Pipe 

from  Sludge  Beds 
i-'l"J 


Cross    Section.'' 
FIG.  156. — Grit  Chamber  at  Hamilton,  Ontario. 

Eng.  News,  Vol.  73,  1915,  p.  425. 


400  SCREENING   AND  SEDIMENTATION 

without  stopping  the  sewage  flow.  The  details  of  the  chamber 
at  Hamilton,  Ontario,  are  shown  in  Fig.  156.  In  studying  these 
drawings  the  following  features  should  be  noted:  1st,  the  smooth 
curves  in  the  channel  to  prevent  eddies,  undue  deposition  of 
organic  matter,  and  difficulties  in  cleaning;  2nd,  the  hopper  in  the 
upper  end  of  the  grit  storage  chamber  and  the  slope  of  the  bot- 
tom of  at  least  1 : 20 ;  and  3rd,  the  simplicity  of  the  inlet  and  out- 
let devices  which  may  be  either  stop  planks  or  cast  iron  sluice 
gates. 

The  drawings  shown  are  merely  representative  of  some  sat- 
isfactory7 types.  The  number  and  variety  of  grit  chambers 
in  existence  is  great.  In  designing  grit  chambers  consideration 
must  be  given  to  the  method  of  cleaning.  They  are  ordinarily 
cleaned  by  such  methods  as  have  been  described  for  the  cleaning 
of  catch-basins  in  Chapter  XII.  Continuous  bucket  scrapers 
similar  to  excavating  machines  are  sometimes  used  for  the  clean- 
ing of  large  grit  chambers.  The  period  between  cleanings  is 
variable.  The  design  should  be  such  as  not  to  require  more 
frequent  cleanings  than  twice  a  month  under  the  worst  conditions. 
The  fluctuations  in  quality  and  quantity  of  grit  will  vary  the  period 
between  cleanings. 

238.  Number  of  Grit  Chambers. — The  period  of  retention 
in  grit  chambers  is  so  short  and  the  velocity  of  flow  so  near  the 
maximum  and  minimum  limitations  that  the  wide  fluctuations 
in  the  rate  of  discharge  in  storm  and  combined  sewers  necessi- 
tates the  construction  of  a  number  of  chambers  which  should 
be  operated  in  parallel  in  order  to  maintain  the  velocity  between 
th^  proper  limits.  Unless  arrangements  are  made  permitting 
the  cleaning  of  grit  chambers  during  operation,  more  than  one 
grit  chamber  should  be  installed  in  order  that  when  one  is  being 
cleaned  the  others  may  be  in  operation.  The  number  of  grit 
chambers  must  be  determined  by  the  desired  conditions  of 
operation  and  the  cost  of  construction.  The  larger  the  number 
of  basins  the  more  nearly  the  flow  in  any  one  basin  can  be 
maintained  constant,  but  the  more  expensive  the  construction. 
The  increase  in  velocity  of  flow  with  increasing  quantity  is 
dependent  on  the  outlet  arrangements.  In  a  shallow  chamber 
with  vertical  sides  and  a  standard  sharp-crested  rectangular 
weir  at  the  outlet  the  velocity  will  vary  approximately  as  the  cube 
root  of  the  rate  of  flow.  Similarly  if  the  outlet  is  a  V  notch  the 


QUANTITY  AND  CHARACTERISTICS  OF  SLUDGE        401 

velocity  will  vary  as  the  fifth  root  of  the  rate  of  flow.  In  all 
cases  the  deeper  the  basin  the  more  nearly  the  velocity  varies 
directly  as  the  rate  of  flow.  The  outlet  weir  can  be  arranged 
as  at  Cleveland,  so  that  the  velocity  remains  constant  for  all 
rates  of  flow  within  certain  limits.  It  is  seldom  that  more  than 
three  grit  chambers  are  necessary  to  care  for  the  fluctuations 
in  flow. 

239.  Quantity    and    Characteristics    of    Sludge    from  Plain 
Sedimentation. — The  sludge  removed  from  plain  sedimentation 
basins  is  slimy,  offensive,  not  easily  dried,  and  is  highly  putres- 
cible  and  odoriferous.     It  contains  about  90  per  cent  moisture 
and   has   a   specific   gravity   from    1.01   to    1.05.     The   amount 
removed  varies  between  2  and  5  cubic  yards  per  million  gallons 
of    sewage.     The    percentage    of    suspended    matter    removed 
varies  between  20  and  60.     The  total  amount  removed  and  the 
percentage   removal  depend   on  the    character  of  the   sewage, 
the  type  of  basin,  and  the  period  of  detention. 

240.  Dimensions  of  Sedimentation  Basins. — The  dimensions 
of  a  sedimentation  basin  are  determined  by  a  method  similar 
to  the  one  given  for  the  determination  of  the  dimensions  of  a 
grit  chamber  in  Art.  236.  .  The  capacity  of  the  basin  is  first 
fixed  upon  to  give  the  required  period  of  sedimentation  and 
sludge  storage  capacity.     The  length  of  the  basin  is  the  product 
of  the  velocity  and  the  period  of  retention.     The  length,  width, 
and  depth  of  the  basin  are  normally  fixed  by  considerations  of 
economy  and  the  limitations  of  the  local  conditions,   such  as 
available  area,  topography,  foundations,  etc.,  and  examples  of 
good   practice.     A  study  of  basins  in  use  shows  the  relation 
between  length  and  width  to  vary  normally  between  2:1  and  4:1. 
Widths  greater  than  30  to  50  feet  are  undesirable  because  of  the 
danger  of  cross  currents  and  back  eddies  which  will  reduce  the 
efficiency  of  the  sedimentation.     Depths  used  in  practice  vary 
too  widely  to  act  as  guides  for  any  particular  design.     Theo- 
retically the  shallower  the  basin  the  better  the  result.     Tanks 
abroad  have  been  built  as  shallow  as  3  feet  and  some  in  this 
country  as  deep  as  16  feet.     The  economical  dimensions  can  be 
determined  by  trial  or  by  calculus.     They  will  serve  as  a  guide 
in  the  adoption  of  the  final  dimensions. 

The  method  to  be  pursued  in  determining  the  economical 
dimensions  of  any  engineering  structure  are : 


402 


SCREENING  AND  SEDIMENTATION 


•  b—- 


I.  Express  the  total  cost  of  the  structure  in  terms  of 
as  few  variables  as  possible. 

II.  Express  all  of  the  variables  in  terms  of  any  one  and 
rewrite  the  expression  for  the  total  cost  in  terms  of  this 
one  variable. 

III.  Equate  the  first  derivative  of  the  expression  with 
regard  to  this  variable  to  zero  and  solve  for  the  variable. 
The  result  will  be  the  economical  value  of  the  variable. 
The  values  of  the  other  variables  can  be  computed  from  the 
relations  already  expressed. 

For  example,  let  it  be  desired  to  determine  the  dimen- 
sions of  two  continuous-flow  sedimentation  basins  as  shown 
in  Fig.  157,  in  which  the 
period  of  retention  in  each 
is  to  be  2  hours,  the  veloc-  ~* 
ity  of  flow  is  not  to  ex- 
ceed one  foot  per  second,  i 
and  the  sludge  accumula-  1 
ted  will  be  3  cubic  yards 
per  million  gallons  of  sew-  i 
age  treated.  The  quantity 
of  sewage  to  be  treated  "* 
is  18,000,000  gallons  per  FlG-  157.— Diagram  for  the  Compu- 
day.  The  shortest  time  tation  of  Economical  Basin  Di- 
between  cleanings  will  be  mensions. 
2  weeks. 

The  capacity  of  each  basin  must  be  2/24  of  18,000,000 
gallons,  or  200,000  cubic  feet  in  order  to  allow  a  period 
of  retention  of  2  hours.  To  this  volume  should  be  added 
sufficient  capacity  to  allow  for  the  2  weeks  of  sludge  stor- 
age between  cleanings.  When  a  basin  is  being  cleaned 
the  load  must  be  put  on  the  remaining  basins.  Then  if 
Q  represents  the  rate  of  accumulation  of  sludge  per  day, 
n  represents  the  number  of  days  between  cleanings,  m 
represents  the  number  of  basins,  and  s  the  sludge  capacity 
of  one  basin,  then 

s=Q(n-l)  ,     Q 

m         m—l' 

The  sludge  storage  capacity  for  the  example  given 
will  be  approximately  11,000  cubic  feet. 

In  expressing  the  total  cost  of  the  basins  let 

h  =  the  depth  in  feet. 
I  =the  length  in  feet. 
b  =the  width  in  feet. 


DIMENSIONS  OF  SEDIMENTATION  BASINS  403 

The  cost  of  land,  floor,  etc.,  per  square  foot    =p  dollars. 
The  cost  of  wall  per  foot  length  =qh2  dollars. 

The  cost  of  pipes,  valves  and  appurtenances  =  P  dollars. 

Then  the  total  cost  C  =  (3l+4b)qh2+2plb+P. 

It  is  now  necessary  to  express  the  three  variables  6,  Z, 
and  h,  in  terms  of  one  of  them.  From  the  relation  Q  = 
2blh  it  is  possible  to  rewrite  the  expression  for  the  total 
cost  as: 


Holding  h  constant  and  differentiating  with  regard  to 
6  in  the  first  expression  and  with  regard  to  I  in  the  second 
expression,  equating  to  zero  and  solving  we  get: 


-V3  an     -VI- 

The  economical  relation  between  6  and  I  is  therefore 
6=0.75? 

regardless  of  the  value  of  h. 

Substituting  these  values  of  I  and  b  in  the  original 
expression  for  the  total  cost,  it  becomes 


Differentiating  with  regard  to  h,  equating  to  zero,  and 
solving 


g  / 

In  the  example  given  if  q  =0 . 2  and  p  =  1 . 0  then 
h  =  11 . 6  feet,  6  =  120  feet  and  I  =  160  feet. 

Since  these  are  reasonable  dimensions  and  in  accord  with  good 
practice  they  should  be  used,  unless  other  conditions  are  unsuit- 
able or  the  velocity  of  flow  is  too  great.  A  width  of  channel  of 
120  feet  as  compared  to  a  length  of  160  feet  is  conducive  to  a 
poor  distribution  of  velocity  across  the  basin.  A  ratio  of  width 
to  length  of  about  1:4  is  desirable.  In  this  case,  by  the  use 
of  three  baffles  parallel  to  the  length  of  the  basin,  thus  dividing 
it  into  channels  40  feet  wide  and  11.6  feet  deep,  the  ratio  of 
width  to  length  is  changed  to  1 : 4  and  the  velocity  will  be 


404 


SCREENING  AND  SEDIMENTATION 


increased  only  to  0.06  foot  per  second  or  3.6  feet  per  minute, 
which  is  a  reasonable  velocity.  It  could  be  reduced  by  increasing 
the  spacing  of  the  baffles  or  the  depth  of  the  chamber. 

Complicated  baffling  is  undesirable.  Two  or  three  overflow 
baffles  may  be  used  to  permit  quiescent  sedimentation  in  the 
space  thus  formed,  and  hanging  baffles  may  be  placed  before 
the  inlet  and  outlet  to  break  up  surface  currents,  or  to  prevent 
the  movement  of  scum.  The  hanging  baffles  should  not  extend 
more  than  12  to  18  inches  below  the  water  surface.  The  inlet 
and  outlet  are  sometimes  arranged  to  permit  the  reversal  of  flow, 
and  the  connecting  channels  between  basins  to  allow  the  opera- 
tion of  any  number  of  basins  in  series  or  in  parallel,  although 
such  arrangements  are  more  important  in  water  purification. 
Sewage  should  enter  and  leave  at  the  top  of  the  basin. 

Cleaning  is  facilitated  by  the  location  of  a  central  gutter  in 
the  bottom  of  the  basin  with  the  slope  of  the  bottom  of  the  basin 
towards  the  gutter  from  1  :  25  to  1  :  80  or  steeper.  A  pipe, 
2  inches  or  larger  in  diameter,  containing  water  under  pres- 
sure with  connections  for  hose  placed  at  frequent  intervals  is  a 
useful  adjunct  in  flushing  the  sludge  from  the  sedimentation 
basins.  For  equal  capacity,  deer  vertical  flow  tanks  are  more 

expensive  and  difficult  to  con- 
struct than  the  shallower  rect- 
angular type.  Deep  tanks  are 
advantageous,  however,  in  that 
sludge  can  sometimes  be  re- 
moved by  gravity  or  by  pump- 
ing without  stopping  the  opera- 
tion of  the  tank.  They  will 
also  operate  successfully  with 
shorter  periods  of  detention 
and  higher  velocities.  The  up- 
ward velocity  should  not  be 
greater  than  the  velocity  of 
sedimentation  of  the  smallest 
particle  to  be  removed.  The 
them  will  be  increased  by  the 


FIG.  158. — Section  through  a  Dort- 
mund Tank. 
Depth  20  to  30  feet. 

efficiency  of    sedimentation  in 


sedimentation  of  the  larger  particles  which  drag  some  of  the 
smaller  particles  down  with  them.  The  Dortmund  tank  shown 
in  Fig.  158  is  an  example  of  this  type. 


CHEMICALS  405 

Ordinarily  it  is  not  necessary  to  roof  sedimentation  basins 
as  the  odors  created  are  not  strong,  and  difficulties  with  ice  are 
seldom  serious. 

CHEMICAL  PRECIPITATION 

241.  The  Process. — Chemical  precipitation  consists  in  adding 
to  the  sewage  such  chemicals  as  will,  by  reaction  with  each  other 
and  the  constituents  of  the  sewage,  produce  a  flocculent  precipi- 
tate and  thus  hasten  sedimentation.     The  advantages  of  this 
process  over  plain  sedimentation  are  a  more  rapid  and  thorough 
removal   of   suspended   matter.     Its   disadvantages   include  the 
accumulation   of   a   large   amount   of   sludge,  the  necessity   for 
skilled  attendance,  and  the  expense  of  chemicals.     The  process 
is    not    in    extensive    use    as    the    conditions    under    which  the 
advantages   outweigh    the  disadvantages  are  unusual.     Sewage 
containing  large  quantities  of  substances  which  will  react  with  a 
small  amount  of  an  added   chemical  to  produce  the  required 
precipitate  are  the  most  favorable  for  this  method  of  treatment. 

Chemical  precipitation  accomplishes  the  same  result  as  plain 
sedimentation,  although  the  effluent  from  the  chemically  pre- 
cipitated sewage  may  be  of  better  quality  than  that  from  a  plain 
sedimentation  basin. 

242.  Chemicals. — Lime  is  practically  the  only  chemical  used 
for  the  precipitation  of  the  solid  matter  in  sewage.     Commercial 
lime   used   for   precipitation   consists   of   calcium   oxide   (CaO), 
with  large  quantities  of  impurities.     It  should  be  stored  in  a  dry 
place  and  protected  from  undue  exposure  to  the  air  to  prevent 
the  formation  of  calcium  carbonate  (CaCOs),  the  formation  of 
which  is  commonly  known  as  air  slacking.     The  active  work 
in  the  formation  of  the  precipitate  is  performed  by  the  lime  (CaO) 
or  calcium  hydroxide    (Ca(OH)2).     The  lime   should   therefore 
be  purchased  on  the  basis  of  available  CaO,  which  may  be  as 
low  as  10  to  15  per  cent  in  some  commercial  products.     The 
amount  of  lime  necessary  depends  on  the  quality  of  the  sewage, 
the  period  of  retention  in  the  sedimentation  basin,  the  method 
of  application,  the  required  results,  and  other  less  easily  measured 
factors.     Full   scale   tests  for  the   amount   of   lime   needed   to 
produce  certain  results  are  the  most  satisfactory.     In  practice 
the  amount  of  lime  necessary  when  lime  alone  is  used  as  a  pre- 


406  SCREENING  AND  SEDIMENTATION 

cipitant  has  been  found  to  be  about  15  grains  per  gallon.  This 
may  be  markedly  different,  dependent  on  the  quality  of  the 
sewage.  For  acid  sewages,  lime  alone  is  not  suitable  as  a  pre- 
cipitant since  it  is  necessary  to  add  sufficient  lime  to  neutralize 
the  sewage  before  the  calcium  carbonate  will  be  precipitated. 

The  use  of  copperas  (FeSCU)  together  with  lime,  leads  to 
economy  in  the  use  of  chemicals  as  the  flocculent  precipitate  of 
ferrous  hydroxide  (Fe(OH)2)  is  more  voluminous  than  the 
precipitate  of  calcium  carbonate.  This  is  commonly  known  as 
the  lime  and  iron  process.  The  presence  of  iron  in  certain  trade 
wastes  may  reduce  the  cost  of  chemical  precipitation,  as  the 
necessary  amount  of  copperas  is  reduced.  Where  15  grains  of 
lime  alone  will  be  needed  per  gallon  of  sewage,  the  total  amount 
of  chemicals  used  will  be  reduced  to  8  to  10  grains  per  gallon 
with  the  use  of  lime  and  iron.  This  combination  is  less  expensive 
than  the  use  of  lime  alone,  and  is  even  cheaper  where  the  iron  is 
already  present  in  the  sewage.  Such  a  condition  is  well  illus- 
trated by  the  sewage  at  Worcester,  Mass.,  -where  the  oldest  and 
best  known  chemical  precipitation  plant  in  the  United  States  is 
located.  The  amount  of  lime  used  at  this  plant  has  varied  between 
6  and  10  grains  per  gallon  of  sewage,  the  normal  amount  being 
about  7  grains.  No  iron  is  added  because  of  the  amount  already 
in  solution. 

The  results  of  a  series  of  experiments  on  the  chemical  precipi- 
tation of  sewage  by  Allen  Hazen,  are  given  in  the  1890  Report 
of  the  Massachusetts  State  Board  of  Health,  on  p.  737  of  the 
volume  on  the  Purification  of  Water  and  Sewage.  Hazen  con- 
cludes as  the  result  of  his  experiments:  concerning  lime, 

There  is  a  certain  definite  amount  of  lime  .  .  . 
which  gives  as  good  or  better  results  than  either  more  or 
less.  This  amount  is  that  which  exactly  suffices  to  form 
normal  carbonates  with  all  the  carbonic  acid  of  the 
sewage.  This  amount  can  be  determined  in  a  few  minutes 
by  simple  titration. 

Concerning  lime  and  iron  (copperas)  he  states: 

Ordinary  house  sewage  is  not  sufficiently  alkaline 
to  precipitate  copperas,  and  a  small  amount  of  lime  must 
be  added  to  obtain  good  results.  The  quantity  of  lime 
required  depends  both  upon  the  composition  of  the  sewage 
and  the  amount  of  copperas  used,  and  can  be  calculated 


PREPARATION  AND  ADDITION  OF  CHEMICALS         407 

from  titration  of  the  sewage.  Very  imperfect  results  are 
obtained  from  too  little  lime,  and,  when  too  much  is 
used,  the  excess  is  wasted,  the  result  being  the  same  as 
with  a  smaller  quantity. 

In  precipitation  by  ferric  sulphate  and  crude  alum, 
the  addition  of  lime  was  found  unnecessary,  as  ordinary 
sewage  contains  enough  alkali  to  decompose  these  salts. 
Within  reasonable  limits  the  more  of  these  precipitants 
used  the  better  is  the  result,  but  with  very  large  quanti- 
ties the  improvement  does  not  compare  with  the  increased 
cost. 

Using  equal  values  of  different  precipitants,  applied 
under  the  most  favorable  conditions  for  each,  upon  the 
same  sewage,  the  best  results  were  obtained  from  ferric 
sulphate.  Nearly  as  good  results  were  obtained  from 
copperas  and  lime  used  together,  while  lime  and  alum 
each  gave  somewhat  inferior  effluents.  .  .  .  When  lime 
is  used  there  is  always  so  much  lime  left  in  solution  that 
it  is  doubtful  if  its  use  would  ever  be  found  satisfactory 
except  in  case  of  an  acid  sewage. 

It  is  quite  impossible  to  obtain  effluents  by  chemical 
precipitation  which  will  compare  in  organic  purity  with 
those  obtained  by  intermittent  nitration  through  sand. 

It  is  possible  to  remove  from  one-half  to  two-thirds 
of  the  organic  matter  by  precipitation  .  .  .  and  it  seems 
probable  that  ...  a  result  may  be  obtained  which  will 
effectually  prevent  a  public  nuisance. 

243.  Preparation  and  Addition  of  Chemicals. — Lime  is  not 
readily  soluble  in  water.  Therefore,  it  is  not  best  to  add  the  lime 
as  a  powder  to  the  sewage,  but  to  form  a  milk  of  lime,  that  is, 
a  supersaturated  solution  containing  from  2,000  to;4,000  grains 
per  gallon,  although  dry  slaked  lime  has  sometimes  been  applied 
directly.  The  solution  is  prepared  in  tanks  in  a  quantity  sufficient 
for  some  part  of  the  day's  run,  commonly  sufficient  to  last  through 
one  shift  of  8  or  10  hours.  The  lime  is  prepared  by  placing  the 
amount  necessary  to  fill  one  storage  tank  into  a  slaking  tank 
containing  some  cold  water.  Sufficient  water  is  added  to  keep 
the  solution  just  at  the  boiling  point,  or  steam  may  be  added  to 
make  it  boil.  After  slaking,  it  is  run  into  the  milk-of-Ume  solu- 
tion tank  and  sufficient  water  added  to  bring  to  the  proper 
strength.  The  milk  of  lime  is  added  in  measured  quantities, 
being  controlled  by  a  variable  head  on  a  fixed  orifice  or  weir, 
so  that  it  may  be  varied  with  the  amount  of  sewage  flowing  through 
the  plant.  The  amount  of  lime  to  be  added  is  determined  by 


408 


SCREENING  AND  SEDIMENTATION 


titration  with  phenolphthalein,  experience  indicating  the  color 
to  be  obtained  when  the  proper  amount  of  lime  has  been  added. 
The  use  of  either  copperas  or  alum  has  been  so  rare,  for  the 
precipitation  of  sewage,  that  a  description  of  the  methods  of 
handling  these  chemicals  as  a  sewage  precipitant  is  not  war- 
ranted. An  excellent  description  of  the  methods  of  handling 
these  chemicals  in  water  purification  will  be  found  in  "  Water 
Purification  "  by  Ellms. 


TABLE  81 
RESULTS  OP  CHEMICAL  PRECIPITATION    AT   WORCESTER,    MASSACHUSETTS* 


1900 

1910 

1920 

Amount  of  sewage  treated,  million 
gallons  

4,781 

5,317 

8,893 

Amount     of     sewage     chemically 
treated,  million  gallons  
Gallons  of  wet  sludge  per  million 
gallons  of  sewage  treated 

3,650 

3,574 
4,450 

7,300 
4  185 

Per  cent  of  solids  in  sludge  

4.42 

8  20 

4  64t 

Tons  of  solids 

7,294 

4,182 

6431f 

Pounds  of  lime  added  per  million 
gallons  of  sewage  pumped  
Per  cent  of  organic  matter  removed  : 
By  albuminoid  ammonia: 
Total 

999§ 
52  7t 

762f 
58  4 

534 
51  9 

Suspended  

90.0J 

88.7 

83  6 

By  oxygen  consumed  : 
Total 

62  8t 

61  1 

62  5 

Suspended  

86.  6{ 

89.7 

86  2 

*  Computed  from   Annual   Report   ot    the   Superintendent    of   Sewers,    Nov.    30,  1919, 
and  1920. 

t  These  figures  are  for  1919.     J  These  figures  are  for  1902.      §  These  figures  are  for  1905. 

244.  Results. — The  results  of  Hazen's  experiments  indicate 
that  a  greater  amount  of  suspended  matter  can  be  removed  in 
the  same  time  by  chemical  precipitation  than  by  plain  sedimen- 
tation. The  percentage  of  removal  of  suspended  matter  may  be 
as  high  as  80  to  90  per  cent  with  a  period  of  retention  of  6  to  8 
hours  and  the  addition  of  a  proper  amount  of  chemical.  That 


RESULTS  409 

• 

the  method  is  not  always  a  success  is  shown  by  the  results  of 
some  tests  at  Canton,  Ohio.1     The  report  states: 

....  lime  treatment  removes  about  50  per  cent  of 
the  suspended  matter,  and  in  the  main  about  50  per  cent 
of  the  organic  matter.  .  .  .  These  data  are  instructive 
as  indicating  that  the  addition  of  lime  to  the  Canton 
sewage  in  quantities  as  previously  stated  does  not  materi- 
ally improve  the  character  of  the  resulting  effluent  over 
and  above  that  which  could  be  produced  by  plain  sedi- 
mentation alone. 

The  plant  at  Worcester,  Mass.,  is  the  largest  in  the  United 
States  and  information  from  it  is  of  value.  A  summary  of  the 
results  at  Worcester  for  1900,  1910,  and  1920  are  shown  in  Table 
81. 

1  Report  of  the  Ohio  State  Board  of  Health,  1908,  page  425. 


CHAPTER  XVI 
SEPTICIZATION 

245.  The  Process. — Septic  action  is  a  biological  process  caused 
by  the  activity  of  obligatory  or  facultative  anaerobes  as  the  result 
of  which  certain  organic  compounds  are  reduced  from  higher  to 
lower  conditions  of  oxidation,  some  of  the  solid  organic  substances 
are  rendered  soluble,  and  a  quantity  of  gas  is  given  off.  Among 
these  gases  are:  methane,  hydrogen  sulphide,  and  ammonia. 
The  biologic  process  in  the  septic  tank  represents  the  downward 
portion  of  the  cycle  of  life  and  death,  in  which  complex  organic 
compounds  are  reduced  to  a  more  simple  condition  available 
as  food  for  low  forms  of  plant  life.  The  disposal  of  sewage  by 
septic  action,  when  introduced,  promised  the  solution  of  all 
problems  in  sewage  treatment.  Septic  action  is  now  better 
understood,  and  it  is  known  that  some  of  the  early  claims  were 
unfounded. 

The  principal  advantage  of  septic  action  in  sewage  treatment 
is  the  relatively  small  amount  of  sludge  which  must  be  cared 
for  compared  to  that  produced  by  a  plain  sedimentation  tank. 
The  sludge  from  a  septic  tank  may  be  25  to  30  per  cent  and  in 
some  cases  40  per  cent  less  in  weight,  and  75  to  80  per  cent  less 
in  volume  than  the  sludge  from  a  plain  sedimentation  tank. 
The  most  important  results  of  septic  action  and  the  greatest 
septic  activity  occur  in  the  deposited  organic  matter  or  sludge. 
The  biologic  changes  due  to  septic  action  which  occur  in  the 
liquid  portion  of  the  tank  contents  are  of  little  or  no  importance. 
The  installation  of  a  septic  tank,  although  it  may  fail  to  prevent 
the  nuisance  calling  for  abatement,  has  a  remarkable  psycho- 
logical effect  in  stilling  complaints.  Among  other  advantages 
are  the  comparative  inexpensiveness  of  the  tanks  and  the  small 
amount  of  attention  and  skilled  attendance  required.  The 
tanks  need  cleaning  once  in  6  months  to  a  year.  If  properly 
designed  no  other  attention  is  necessary. 

410 


THE  SEPTIC   TANK  411 

The  septic  tank  has  fallen  into  some  disrepute  because  of  the 
better  results  obtainable  by  other  methods,  the  occasional  dis- 
charge of  effluents  worse  than  the  influent,  the  occasional  dis- 
charge of  sludge  in  the  effluent  caused  by  too  violent  septic  boiling, 
and  on  account  of  patent  litigation.  This  last  difficulty  has  been 
overcome  as  the  Cameron  patents  expired  in  1916.  Occasionally 
the  odors  given  off  by  the  septic  process  are  highly  objectionable 
and  are  carried  for  a  long  distance.  These  odors  can  be  controlled 
to  a  large  extent  by  housing  the  tanks.  Over-septicization  must 
be  guarded  against  as  an  over-septicized  effluent  is  more  difficult 
of  further  treatment  or  of  disposal  than  a  comparatively  fresh, 
untreated  sewage.  An  over-septicized  or  stale  sewage  is  indi- 
cated by  the  presence  of  large  quantities  of  ammonias,  either 
free  or  albuminoid,  frequently  accompanied  by  hydrogen  sul- 
phide and  other  foul-smelling  gases.  The  oxygen  demand  in 
an  over-septicized  sewage  is  greater  than  that  in  a  fresh  or  more 
carefully  treated  sewage. 

246.  The  Septic  Tank. — A  septic  tank  is  a  horizontal,  continu- 
ous-flow, one-story  sedimentation  tank  through  which  sewage 
is  allowed  to  flow  slowly  to  permit  suspended  matter  to  settle 
to  the  bottom  where  it  is  retained  until  anaerobic  decomposition 
is  established,  resulting  in  the  changing  of  some  of  the  suspended 
organic  matter  into  liquid  and  gaseous  substances,  and  a  conse- 
quent reduction  in  the  quantity  of  sludge  to  be  disposed  of.1 
It  is  to  be  noted  that  a  continuous  flow  is  essential  to  a  septic  tank. 
Small  tanks  containing  stagnant  household  sewage  are  called  cess- 
pools, although  sometimes  erroneously  spoken  of  as  septic  tanks. 

Septic  and  sedimentation  tanks  differ  in  their  method  of  opera- 
tion only  in  the  period  of  storage  and  the  frequency  of  cleaning. 
The  period  of  flow  in  a  septic  tank  is  longer  and  it  is  cleaned  less 
frequently.  The  results  obtained  by  the  two  processes  differ 
widely.  A  septic  tank  can  be  converted  into  a  sedimentation 
tank,  or  vice  versa,  by  changing  the  method  of  operation,  no 
constructional  features  requiring  alteration.  The  purpose  of 
the  tank  is  to  store  the  sludge  for  such  a  period  of  time  that 
partial  liquefaction  of  the  sludge  may  take  place,  and  thus 
minimize  the  difficulty  of  sludge  disposal.  For  this  reason  the 
sludge  storage  capacity  of  a  septic  tank  is  sometimes  greater 
than  would  be  necessary  for  a  plain  sedimentation  tank. 
1  Definition  proposed  by  the  Am.  Public  Health  Assn. 


412 


SEPTICIZATION 


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RESULTS  OF  SEPTIC  ACTION  413 

247.  Results  of  Septic  Action. — The  results  obtained  from 
the  septic  tanks  at  the  Columbus  Sewage  Experiment  Station 
are  given  in  Table  82.  The  effluent  is  higher  than  the  influent 
in  free  ammonia,  but  the  reduction  of  other  constituents,  par- 
ticularly suspended  matter,  is  marked. 

Septic  action  is  sensitive  to  temperature  changes,  and  to  cer- 
tain constituents  of  the  incoming  sewage.  Cold  weather  or  an 
acid  influent  will  inhibit  septicization.  In  winter  the  liquefaction 
of  sludge  may  practically  cease,  whereas  in  summer  liquefaction 
may  exceed  deposition.  The  amount  of  gas  generated  is  a 
measure  of  the  relative  amount  of  septic  action.  The  rapid 
generation  of  gas  in  warm  weather  disturbs  the  settled  sludge 
and  may  cause  a  deterioration  of  the  quality  of  the  effluent  because 
of  the  presence  of  decomposed  sludge.  The  results  in  Table  82 
show  the  effect  of  cold  weather  on  the  process.  In  warm  weather 
the  violent  ebullition  of  gas  sometimes  causes  the  discharge  of 
sludge  in  the  effluent,  resulting  in  a  liquid  more  difficult  of 
disposal  than  the  incoming  sewage.  Since  septic  action  is 
dependent  on  the  presence  of  certain  forms  of  bacteria,  where 
these  are  absent  there  will  be  no  septic  action.  Sewage  generally 
contains  the  forms  of  bacteria  necessary  for  this  action  but  it- 
has  occasionally  been  found  necessary  to  seed  new  tanks  in  order 
to  start  septic  action. 

The  sludge  from  septic  tanks  is  usually  black,  with  a  slight 
odor,  though  in  some  cases  this  odor  may  be  highly  offensive. 
The  sludge  will  flow  sluggishly.  It  can  be  pumped  by  centrif- 
ugal pumps  and  it  will  flow  through  pipes  and  channels.  It 
has  a  moisture  content  of  about  90  per  cent  and  a  specific  gravity 
of  about  1.03.  It  is  dried  with  difficulty  on  open-air  drying  beds, 
and  it  is  worthless  as  a  fertilizer.  The  composition  of  some 
septic  sludges  are  shown  in  Table  83. 

248.  Design  of  Septic  Tanks. — The  sedimentation  chambers 
of  a  septic  tank  are  designed  on  the  same  principles  as  the  sedi- 
mentation basins  described  in  Art.  240.  The  velocity  of  flow 
should  not  exceed  one  foot  per  minute.  The  channels  should  be 
straight  and  free  from  obstructions  causing  back  eddies.  The 
ratio  of  length  to  width  of  channel  should  be  between  2  :  1  to  4  :  1 
with  a  width  not  exceeding  50  feet,  and  desirably  narrower. 
The  depths  used  vary  between  5  and  10  feet,  exclusive  of  the 
sludge  storage  capacity.  Hanging  baffles  should  be  placed,  one 


414 


SEPTICIZATION 


before  the  inlet  and  the  other  in  front  of  the  outlet,  so  as  to 
distribute  the  incoming  sewage  over  the  tank,  and  to  prevent 
scum  from  passing  into  the  outlet.  The  baffles  should  hang 
about  12  inches  below  the  surface  of  the  sewage.  Intermediate 
baffles  are  sometimes  desirable  to  prevent  the  movement  of 
sludge  or  scum  towards  the  outlet.  The  placing  of  baffles  must 
be  considered  carefully  as  injudicious  baffling  may  lessen  the 
effectiveness  of  a  tank  by  so  concentrating  the  currents  as  to  pre- 
vent sedimentation  or  the  accumulation  of  sludge.  Baffles 
should  be  built  of  concrete  or  brick,  as  wood  or  metal  in  contact 
with  septic  sewage  deteriorates  rapidly.  In  designing  the  sludge 
storage  chambers  it  may  be  assumed  that  one-half  of  the  organic 
matter  and  none  of  the  mineral  matter  will  be  liquefied  or  gasi- 
fied. The  net  storage  volume  allowed  is  about  2  to  3  cubic  yards 
per  million  gallons  of  sewage  treated.  Variations  between  0.1 
and  10.0  cubic  yards  have  been  recorded,  however.  If  grit  is 
carried  in  the  sewage  to  be  treated,  it  should  be  removed  by 
the  installation  of  a  grit  chamber  before  the  sewage  enters  the 
septic  tank. 

TABLE  83 
ANALYSIS  OF  TANK  SLUDGES 


2 

Per  Cent  in  Terms 

s 

c 

.1? 

to 

of  Dry  Matter 

£3 

K* 

'o 

P<  0$ 

• 

2 

^ 

T)O 

t<O 

Kind  of 

Place 

O 

o 

I 

* 

31 

O 

rogen 

?l- 

II  & 

Sludge 

Reference 

D. 

S3 

o 

X 

."S 

~§ 

*§S^ 

o 

QQ 

£ 

Z* 

fc 

O 

PH 

f 

1908         Report, 

Mansfield   O 

1    11 

80  8 

Septic      i 

State  Board  of 

Health 

Chicago,  111  

1.03 

90 

40 

60 

1.9 

7.0 

1.0 

200 

Septic 

1.5 

300 

Columbus,  O.  .  . 

1.09 

83.3 

4.4 

16.7 

0.25 

0.94 

Septic      < 

G.    A.    Johnson 
1905  Report 

Atlanta,  Ga.... 

1.02 

87.1 

39.1 

60.9 

1.25 

6.11 

Imhoff    < 

Eng.  Rec.f  V.  72, 
1915   p.  4 

Baltimore,  Md. 

1.02 

91.9 

i 

66.2 

2.45 

4.02 

...j 

Diges-     ] 
tion        [ 
Tank    J 

Eng.  News-Rec., 
V.  87,  1921,  p.  98 

Baltimore,  Md  . 

1.02 

92.4 

62.7 

1 

2.75 

Imhoff 

do. 

Baltimore,  Md. 

79.2 

73.8 

2.64 

9.00 

/ 

Raw         \ 
Sludge     / 

do. 

j 

Baltimore,  Md. 

92.4 

58.0 

3.19 

-1 

Settling  1 
Basin       / 

do. 

DESIGN  OF  SEPTIC   TANKS  415 

Two  or  more  tanks  should  be  constructed  to  allow  for  the  shut 
down  of  one  for  cleaning  and  to  increase  the  elasticity  of  the 
plant.  The  number  of  tanks  to  be  used  is  dependent  on  the 
total  quantity  of  sewage  and  the  fluctuations  in  rate  of  flow.  An 
average  period  of  retention  of  about  9  to  10  hours  with  a  mini- 
mum period  of  6  hours  during  maximum  flow  is  a  fair  average 
to  be  assumed  for  design.  The  period  of  retention  should  not 
exceed  about  24  hours,  as  the  sewage  may  become  over-septi- 
cized.  The  sludge  storage  period  should  be  from  6  to  12  months. 

A  cover  is  not  necessary  to  the  successful  operation  of  a  septic 
tank.  Covers  are  sometimes  used  with  success,  however,  in 
reducing  the  dissemination  of  odors  from  the  tank.  They  are 
also  useful  in  retaining  the  heat  of  the  sewage  in  cold  weather 
and  thus  aid  in  promoting  bacterial  activity.  Types  of  covers 
vary  from  a  building  erected  over  the  tank  to  a  flat  slab  set  close 
to  the  surface  of  the  sewage.  In  the  design  of  a  cover,  good 
ventilation  should  be  provided  to  permit  the  escape  of  the  gases, 
and  easy  access  should  be  provided  for  cleaning.  Tightly 
covered  tanks  or  tanks  with  too  little  ventilation  have  resulted 
in  serious  explosions,  as  at  Saratoga  Springs  in  1906  and  at 
Florenceville,  N.  C.,  in  1915.1 

The  sludge  may  be  removed  through  drains  in  the  bottom  of 
the  tank  as  described  for  sedimentation  basins,  or  where  such 
drains  are  not  feasible  the  sludge  and  sewage  are  pumped  out. 
For  this  purpose  a  pump  may  be  installed  permanently  at  the 
tank,  or  for  small  tanks  portable  pumps  are  sometimes  used. 
Septic  tanks  should  be  cleaned  as  infrequently  as  possible  without 
permitting  the  overflow  of  sludge  into  the  effluent.  The  less 
frequent  the  cleaning  the  less  the  amount  of  sludge  removed 
since  digestion  is  continuous  throughout  the  sludge.  It  is 
necessary  to  clean  when  the  tank  becomes  so  filled  with  sludge, 
that  the  period  of  retention  is  materially  reduced,  or  sludge  is 
being  carried  over  into  the  effluent. 

The  details  of  the  septic  tank  at  Champaign,  Illinois,  are 
shown  in  Fig.  159.  This  tank  was  designed  by  Prof.  A.  N. 
Talbot,  and  was  put  in  service  on  Nov.  1,  1897.  It  was  among 
the  first  of  such  tanks  to  be  installed  in  the  United  States.  The 
tank  shown  in  Fig.  159  is  an  example  of  present  day  practice 
in  single-story  septic  tank  design. 

1  See  Eng.  News,  Vol.  73,  1915,  p.  410. 


416 


SEPTICIZATION 


I Ejector..     ^  %' Steam  Pipe 


Valve 
Chamber 


Plan,  with  Roof  Removed 
FIG.  159. — Septic  Tank  at  Champaign,  Illinois. 

^ Sludge  Outlet 


~~1  —  \  — 

1  1 

r—  n  

V^/fr  Samp 

T 

k-  u 

J 

r~ 

1 

~1 

|~D  \i 

~n  —  i  — 

1 

_^ 

<-  

8-    «... 

...j 

Plan 


f< 8-0- ->( 

Section. 

FIG.  160.— Design  for  a  Residential  Septic  Tank  for  a  Family  of  Ten.     Illinois 
State  Board  of  Health. 


IMHOFF  TANKS 


417 


Small  septic  tanks  for  rural  homes  of  5  to  15  persons,  Or  on 
a  slightly  larger  scale  for  country  schools  and  small  institutions, 
are  little  more  than  glorified  cesspools.  Nevertheless  much 
attention  has  been  given  to  the  construction  of  such  tanks  by 
the  National  Government  and  by  state  boards  of  health.1  The 
recommendations  of  some  of  these  boards  have  been  compiled 
in  Table  84.  A  typical  method  for  the  construction  of  such  tanks, 
as  recommended  by  the  Illinois  State  Board  of  Health,  is  shown 
in  Fig.  160.  A  subsurface  filter,  into  which  the  effluent  is  dis- 
charged, is  an  important  adjunct  where  no  adequate  stream  is 
available  to  receive  the  discharge  from  the  tank. 

TABLE  84 
CAPACITIES  OF  SEPTIC  TANKS  FOE  SMALL  INSTALLATIONS 


Rule  Recommended  by 
State  Board  of  Health 

Number, 
Persons 

Capacity, 
Gallons 
per  Person 

Period  of 
Retention 

Remarks 

Wisconsin  
Ohio  .      .  .            

4  to  10 

30 
50 

24  hours 

Not  less  than  560  gallons 

24  to  48  hours 

Texas     

24  hours 

Illinois  
U.  S.  Dept.  Agriculture. 

North  Carolina  

Large 

45 
40 

1       15 

24  hours 
24  hours 

25    per    cent    additional 
capacity  for  sludge 

Not  less  than  500  gallons 

North  Carolina  
North  Carolina  

North  Carolina  

Schools 
20  pupils 
Medium 
School 
Homes 

/ 
25 

}    20 

25  to  30 

249.  Imhoff  Tanks. — In  the  discussion  of  septic  tanks  it  has 
been  brought  out  that  one  of  the  objections  to  their  use  is  the 
unloading  of  sludge  into  the  effluent  which  occasionally  causes 
a  greater  amount  of  suspended  matter  iri  the  effluent  than  in  the 
influent.  The  Imhoff  tank  is  a  form  of  septic  tank  so  arranged 
that  this  difficulty  is  overcome.  It  combines  the  advantages 
of  the  septic  and  sedimentation  tanks  and  overcomes  some  of  their 
disadvantages.  An  Imhoff  tank  is  a  device  for  the  treatment 
of  sewage,  consisting  of  a  tank  divided  into  3  compartments. 
The  upper  compartment  is  called  the  sedimentation  chamber.  In 

1  Sewage  Treatment  from  Single  Houses  and  Small  Communities,  by  L. 
C.  Frank.  U.  S.  Public  Health  Service,  Bulletin  101,  1920. 


418 


SEPTICIZATION 


it  the  sedimentation  of  suspended  solids  causes  them  to  drop 
through  a  slot  in  the  bottom  of  the  chamber  to  the  lower  com- 
partment called  the  digestion  chamber.  In  this  chamber  the  solid 
matter  is  humified  by  an  action  similar  to  that  in  a  plain  septic 
tank.  The  generated  gases  escape  from  the  digestion  chamber  to 
the  surface  through  the  third  compartment  called  the  transition  or 
scum  chamber.  Sections  of  Imhoff  tanks  are  shown  in  Fig.  161. 
It  is  essential  to  the  construction  of  an  Imhoff  tank  that  the  slot 
in  the  bottom  of  the  sedimentation  chamber  does  not  permit 


Effect  of 
Design  on 
Sludge-Storage 

Capacity. 

(AandB) 


Original   Design    of  Imhoff  lank 


Downward  and  Upward 
FlowTank 

FIG.  161.— Typical  Sections  through  Imhoff  Tanks. 

Eng.  News,  Vol.  75,  p.  15. 

the  return  of  gases  through  the  sedimentation  chamber,  and 
that  there  be  no  flow  in  the  digestion  chamber. 

The  Imhoff  tank  was  invented  by  Dr.  Karl  Imhoff,  director 
of  the  Emscher  Sewerage  District  in  Germany.  Its  design  is 
patented  in  the  United  States,  the  control  of  the  patent  being 
in  the  hands  of  the  Pacific  Flush  Tank  Co.  of  Chicago,  which 
collects  the  royalties  which  are  payable  when  construction  work 
begins.  The  fee  for  a  tank  serving  100  persons  is  $10,  for  1,000 
persons  is  $80  and  for  100,000  persons  is  $2550.  The  rate  of 
the  royalty  reduces  in  proportion  as  the  number  of  persons  served 
increases.1  As  designed  by  Imhoff  and  used  in  Germany  the  tanks 
were  of  the  radial  flow  type  and  quite  deep.  The  depth,  as 
1  Eng.  News  Record,  Vol.  78,  1917,  p.  566. 


DESIGN  OF  IMHOFF  TANKS 


419 


explained  by  Imhoff,  is  one  of  the  chief  requirements  for  the 
successful  operation  of  the  tank.  As  adapted  to  American 
practice  the  tanks  are  generally  of  the  longitudinal  flow  type 
and  are  not  made  so  deep.  An  isometric  view  of  a  radial  flow 
Imhoff  tank  is  shown  in  Fig.  162.  The  sewage  enters  at  the 
center  of  the  tank  near  the  surface  and  flows  radially  outward 
under  the  scum  ring  and  over  a  weir  placed  near  the  circum- 
ference of  the  tank.  One  type  of  longitudinal  flow  tank  is  shown 
in  isometric  view  in  Fig.  163. 


-.r 

r~Sludqe  Outlet 
Pipe 


Channelto 
Sludge  Bed 

\Sedimenfation  Comp'f. 


\&m5™°.m- 40- 

Weirs^      Gas  Stack 
Distribut ' 
Cha' 


-  Sludge  Corfip't 


ff* 

''Gravel 


FIG.  162. — Sketch  of  Radial  Flow  Imhoff  Tank  at  Baltimore,  Maryland. 

Eng.  Record,  Vol.  70,  p.  5. 

250.  Design  of  Imhoff  Tanks.— The  velocity  of  flow,  period 
of  retention,  and  the  quantity  of  sewage  to  be  treated  determine 
the  dimensions  of  the  sedimentation  chamber  as  in  other  forms 
of  tanks.  The  velocity  of  flow  should  not  exceed  one  foot  per 
minute,  with  a  period  of  retention  of  2  to  3  hours.  A  greater 
velocity  than  one  foot  per  minute  results  in  less  efficient  sedi- 
mentation. A  longer  period  of  retention  than  the  approximate 
limit  set  may  result  in  a  septic  or  stale  effluent,  and  a  shorter 
period  may  result  in  loss  of  efficiency  of  sedimentation.  The 
bottom  of  the  sedimentation  chamber  should  slope  not  less 
than  1 J  vertical  to  1  horizontal,  in  order  that  deposited  material 
will  descend  into  the  sludge  digestion  chamber.  Provision 
should  be  made  for  cleaning  these  sloping  surfaces  by  placing 
a  walk  on  the  top  of  the  tank  from  which  a  squeegee  can  be 
handled  to  push  down  accumulated  deposits.  It  is  desirable 
to  make  the  material  of  the  sides  and  bottom  of  the  sedimenta- 


420 


SEPTICIZATION 


tion  chamber  as  smooth  as  possible  to  assist  in  preventing  the 
retention  of  sludge  in  the  sedimentation  chamber.  Wood,  glass, 
and  concrete  have  been  used.  The  latter  is  the  more  common 
and  has  been  found  to  be  satisfactory.  The  length  of  the  sedi- 
mentation chamber  is  fixed  by  the  velocity  of  flow  and  the 
period  of  retention.  Tanks  are  seldom  built  over  100  feet  in 


'Stop  Plank         ^  Outlet  Channel  for  Reverse  Flow   f-'-Sfop  Plank 


•Outlet 

.--Tie  Beams 

-- Baffle  Wall 


•*'•"  .'-Equalizing 

-       Port 


InletChannel, 
Direct  Flow  or 
Outlet  Channel, 
Reverse  Flow 


Brace,  10-0" 
C.toC. 


Reinforced 
Concrete 
Walls  and 
Partitions. 


Reducer 

Iron  Supports  —  - 


FIG.  163. — Isometric  View  of  Longitudinal  Flow  Imhoff  Tank  at  Cleburne, 

Texas. 
Eng.  News,  Vol.  76,  p.  1029. 

length,  however,  because  of  the  resulting  unevenness  in  the  accu- 
mulation of  sludge.  Where  longer  flows  are  desired  two  or  more 
tanks  may  be  operated  in  series.  The  width  of  the  chamber 
is  fixed  by  considerations  of  economy  and  convenience.  It 
should  not  be  made  so  great  as  to  permit  cross  currents.  In 
general  a  narrow  chamber  is  desirable.  Satisfactory  chambers 
have  been  constructed  at  depths  between  5  and  15  feet.  The 


DESIGN  OF  IMHOFF  TANKS  421 

depth  of  the  sedimentation  chamber  and  the  depth  of  the  diges- 
tion chamber  each  equal  about  one-half  of  the  total  depth  of  the 
tank.  This  should  be  made  as  deep  as  possible  up  to  a  limit 
of  30  to  35  feet,  with  due  consideration  of  the  difficulties  of 
excavation.  C.  F.  Mebus  states:1 

In  9  of  the  largest  representative  United  States 
installations,  the  depth  from  the  flow  line  to  the  slot 
varies  from  10  feet  10  inches  to  13  feet  6  inches. 

Imhoff  states,  concerning  the  depth  of  tanks: 

Deep  tanks  are  to  be  preferred  to  shallow  tanks 
because  in  them  the  decomposition  of  the  sludge  is 
improved.  This  is  so  because  in  the  deeper  tanks  the 
temperature  is  maintained  more  uniformly  and  because 
the  stirring  action  of  the  rising  gas  bubbles  is  more 
intense. 

The  stirring  action  of  the  gas  bubbles  is  desirable  as  it  brings  the 
fresh  sludge  more  quickly  under  the  influence  of  the  active  bac- 
terial agents.  The  greater  pressure  on  the  sludge  in  deep  tanks 
also  reduces  its  moisture  content. 

Two  or  more  sedimentation  chambers  are  sometimes  used  over 
one  sludge  digestion  chamber  in  order  to  avoid  the  depths  called 
for  by  the  sloping  sides  of  a  single  sedimentation  chamber.  An 
objection  to  multiple-flow  chambers  is  the  possibility  of  interchange 
of  liquid  from  one  chamber  to  another  through  the  common 
digestion  chamber. 

The  inlet  and  outlet  devices  should  be  so  constructed  that 
the  direction  of  flow  in  the  tank  can  be  reversed  in  order  that  the 
accumulated  sludge  may  be  more  evenly  distributed  in  the  hop- 
pers of  the  digestion  chamber.  The  sewage  should  leave  the 
sedimentation  chamber  over  a  broad  crested  weir  in  order  to 
minimize  fluctuations  in  the  level  of  sewage  in  the  tank.  The 
gases  in  the  digesting  sludge  are  sensitive  to  slight  changes  in 
pressure.  A  lowering  of  the  level  of  sewage  will  release  com- 
pressed gas  and  will  too  violently  disturb  the  sludge  in  the 
digestion  chamber.  Hanging  baffles,  submerged  12  to  16  inches 
and  projecting  12  inches  above  the  surface  of  the  sewage,  should 
be  placed  in  front  of  the  inlet  and  outlet,  and  in  long  tanks  inter- 
mediate baffles  should  be  placed  to  prevent  the  movement  of 
1  Municipal  Engineering,  Vol.  54,  p.  149. 


422  SEPTICIZATION 

scum  or  its  escape  into  the  effluent.  An  Imhoff  tank  which  is 
operating  properly  should  not  have  any  scum  on  the  surface  of 
the  sewage  in  the  sedimentation  chamber. 

The  slot  or  opening  at  the  bottom  of  the  sedimentation 
chamber  should  not  be  less  than  6  inches  wide  between  the  lips. 
Wider  slots  are  preferable,  but  too  wide  a  slot  will  involve  too 
much  loss  of  volume  in  the  digestion  chamber.  One  lip  of  the 
slot  should  project  at  least  3  inches  horizontally  under  the  other 
so  as  to  prevent  the  return  of  gases  through  the  sedimentation 
chamber.  A  triangular  beam  may  be  used  as  shown  in  Fig.  161  A. 
This  method  of  construction  is  advantageous  in  increasing  the 
available  capacity  for  sludge  storage. 

The  digestion  chamber  should  be  designed  to  store  sludge  from 
6  to  12  months,  the  longer  storage  periods  being  used  for  smaller 
installations.  In  warm  climates  a  shorter  period  may  be  used 
with  success.  The  amount  of  sludge  that  will  be  accumulated 
is  as  uncertain  as  in  other  forms  of  sewage  treatment.  A  widely 
quoted  empirical  formula,  presented  in  "  Sewage  Sludge "  by 
Allen,  states: 

C  =  10 . 5    P  D  for  combined  sewage; 
C  =  5 . 25  P  D  for  separate  sewage, 

in  which  C  =  the    effective  capacity  of  the  digestion    chamber 

in  cubic  feet; 

P=the  population  served,  expressed  in  thousands; 
D  =  the  number  of  days  of  storage  of  sludge. 

The  effective  capacity  of  the  chamber  is  measured  as  the  entire 
volume  of  the  chamber  approximately  18  inches  below  the 
lower  lip  of  the  slot.  The  capacity  as  computed  from  the  above 
formula  is  assumed  as  satisfactory  for  a  deep  tank.  Frank 
and  Fries  1  recommend  the  increase  of  the  capacity  for  shallow 
tanks  to  compensate  for  the  decreased  hydrostatic  pressure. 
In  any  event  the  formula  can  be  no  more  than  a  guide  to  design. 
No  formula  can  be  of  equal  value  to  data  accumulated  from 
tests  on  the  sewage  to  be  treated.  The  Illinois  State  Board  of 
Health  requires  3  cubic  yards  of  sludge  digestion  space  per 
million  gallons  of  sewage  treated.  Frank  and  Fries  recommend 
an  allowance  of  0.007  cubic  foot  of  storage  per  inhabitant  per  day 
for  combined  sewage  and  one-half  that  amount  for  separate 
1  Eng.  Record,  Vol.  68,  1913,  p.  452. 


DESIGN  OF  IMHOFF  TANKS  423 

sewage.  If  this  is  based  on  80  per  cent  moisture  content,  the 
volume  for  other  percentages  of  moisture  can  be  easily  com- 
puted. An  average  figure  used  in  the  Emscher  District  is  one 
cubic  foot  capacity  for  each  inhabitant  for  the  combined  system, 
and  three-fourths  of  this  for  the  separate  system.  Metcalf 
and  Eddy 1  recommend  the  following  method  for  the  deter- 
mination of  the  sludge  storage  capacity:  (1)  From  analyses  of 
the  sewage  or  study  of  the  sources  ascertain  the  amount  of  sus- 
pended matter.  (2)  Assume,  or  determine  by  test,  the  amount 
which  will  settle  in  the  period  of  detention  selected,  say  60  per 
cent  in  3  hours.  (3)  Estimate  the  amount  which  will  be  digested 
in  the  sludge  chamber  at  about  25  per  cent,  leaving  75  per  cent 
to  be  stored.  (4)  Estimate  the  percentage  moisture  in  the  sludge 
conservatively,  say  85  per  cent.  The  total  volume  of  sludge 
can  then  be  computed.  This  method  is  more  rational  than 
the  use  of  empirical  formulas,  but  because  of  the  estimates 
which  must  be  made  its  results  will  probably  be  of  no  greater 
accuracy  than  those  obtained  empirically. 

The  digestion  chamber  is  made  in  the  form  of  an  inverted 
cone  or  pyramid  with  side  slopes  at  most  about  2  horizontal  to 
1  vertical  and  preferably  much  steeper  without  necessitating  too 
great  a  depth  of  tank.  The  purpose  of  the  steep  slope  is  to  con- 
centrate the  sludge  at  the  bottom  of  the  hopper  thus  formed. 
Concrete  is  ordinarily  used  as  the  material  of  construction  as 
a  smooth  surface  can  be  obtained  by  proper  workmanship. 
Where  flat  slopes  have  been  used,  a  water  pipe  perforated  at 
intervals  of  6  to  12  inches  may  be  placed  at  the  top  of  the  slopes, 
and  water  admitted  for  a  short  time  to  move  the  sludge  when  the 
tank  is  being  cleaned. 

A  cast-iron  pipe,  6  to  8  inches  in  diameter,  is  supported  in  an 
approximately  vertical  position  with  its  open  lower  end  supported 
about  12  inches  above  the  lowest  point  in  the  digestion  chamber. 
This  is  used  for  the  removal  of  sludge.  A  straight  pipe  from  the 
bottom  of  the  tank  to  a  free  opening  in  the  atmosphere  is  desir- 
able in  order  to  allow  the  cleaning  of  the  pipe  or  the  loosening 
of  sludge  at  the  start,  and  to  prevent  the  accumulation  of  gas 
pockets.  The  sludge  is  led  off  through  an  approximately  hori- 
zontal branch  so  located  that  from  4  to  6  feet  of  head  are  available 
for  the  discharge  of  the  sludge.  A  valve  is  placed  on  the  hori- 
1  Am.  Sewerage  Practice,  Vol.  Ill,  p.  437. 


424  SEPTICIZATION 

zontal  section  of  the  pipe.  A  sludge  pipe  is  shown  in  Fig.  162 
and  163.  Under  such  conditions,  when  the  sludge  valve  is  opened 
the  sludge  should  flow  freely.  The  hydraulic  slope  to  insure 
proper  sludge  flow  should  not  be  less  than  12  to  16  per  cent. 
Where  it  is  not  possible  to  remove  the  sludge  by  gravity  an  air 
lift  is  the  best  method  of  raising  it. 

The  volume  of  the  transition  or  scum  chamber  should  equal 
about  one-half  that  of  the  digestion  chamber.  The  surface  area 
of  the  scum  chamber  exposed  to  the  atmosphere  should  be  25 
to  30  per  cent  of  the  horizontal  projection  of  the  top  of  the 
digestion  chamber.  Some  tanks  have  operated  successfully 
with  only  10  per  cent,  but  troubles  from  foaming  can  usually 
be  anticipated  unless  ample  area  for  the  escape  of  gases  has 
been  provided. 

All  portions  of  the  surface  of  the  tank  should  be  made 
accessible  in  order  that  scum  and  floating  objects  can  be  broken 
up  or  removed.  The  gas  vents  should  be  made  large  enough 
so  that  access  can  be  gained  to  the  sludge  chamber  through 
them  when  the  tank  is  empty. 

Precautions  should  be  taken  against  the  wrecking  of  the  tank 
by  high  ground  water  when  the  tank  is  emptied.  With  an  empty 
tank  and  high  ground  water  there  is  a  tendency  for  the  tank  to 
float.  The  flotation  of  the  tank  may  be  prevented  by  building 
the  tank  of  massive  concrete  with  a  heavy  concrete  roof,  by 
underdraining  the  foundation,  or  by  the  installation  of  valves 
which  will  open  inwards  when  the  ground  water  is  higher  than 
the  sewage  in  the  tank.  Dependence  should  not  be  placed  on  the 
attendant  to  keep  the  tank  full  during  periods  of  high  ground 
water. 

Roofs  are  not  essential  to  the  successful  operation  of  Imhoff 
tanks.  They  are  sometimes  used,  however,  as  for  septic  tanks, 
to  assist  in  controlling  the  dissemination  of  odors,  to  minimize 
the  tendency  of  the  sewage  to  freeze,  and  to  aid  in  bacterial 
activity.  In  the  construction  of  a  roof,  ventilation  must  be 
provided  as  well  as  ready  access  to  the  tank  for  inspection, 
cleaning,  and  repairs. 

251.  Imhoff  Tank  Results.— The  Imhoff  tank  has  the 
advantage  over  the  septic  tank  that  it  will  not  deliver  sludge 
in  the  effluent,  except  under  unusual  conditions.  The  Imhoff 
tank  serves  to  digest  sludge  better  than  a  septic  tank  and  it 


STATUS  OF  IMHOFF  TANKS  425 

will  deliver  a  fresher  effluent  than  a  plain  sedimentation  tank. 
Imhoff  sludge  is  more  easily  dried  and  disposed  of  than  the 
sludge  from  either  a  septic  or  a  sedimentation  tank.  This  is 
because  it  has  been  more  thoroughly  humified  and  contains 
only  about  80  per  cent  of  moisture.  As  it  comes  from  the  tank 
it  is  almost  black,  flows  freely  and  is  filled  with  small  bubbles 
of  gas  which  expand  on  the  release  of  pressure  from  the  bottom 
of  the  tank,  thus  giving  the  sludge  a  porous,  sponge-like  consist- 
ency which  aids  in  drying.  When  dry  it  has  a  inoffensive  odor 
like  garden  soil,  and  it  can  be  used  for  filling  waste  land,  without 
further  putrefaction.  It  has  not  been  used  successfully  as  a 
fertilizer. 

Offensive  odors  are  occasionally  given  off  by  Imhoff  tanks, 
even  when  properly  operated.  They  also  have  a  tendency  to 
"  boil  "  or  foam.  The  boiling  may  be  quite  violent,  forcing  scum 
over  the  top  of  the  transition  chamber  and  sludge  through  the 
slot  in  the  sedimentation  chamber,  thus  injuring  the  quality 
of  the  effluent.  The  scum  on  the  surface  of  the  transition  chamber 
may  become  so  thick  or  so  solidly  frozen  as  to  prevent  the  escape 
of  gas  with  the  result  that  sludge  may  be  driven  into  the  sedi- 
mentation chamber. 

Some  chemical  analyses  of  Imhoff  tank  influents  and  efflu- 
ents are  given  in  Table  86  and  the  analyses  of  some  sludges  from 
Imhoff  tanks  are  given  in  Table  83.  It  is  to  be  noted  that  the 
nitrites  and  nitrates  are  still  present  in  the  effluent,  whereas 
they  are  seldom  present  in  the  effluent  from  septic  tanks.  The 
per  cent  of  moisture  in  the  Imhoff  sludge  is  less  than  that  in  the 
septic  tank  sludge,  and  its  specific  gravity  is  higher.  It  is  heavier 
and  more  compact  because  of  the  longer  time  and  the  greater 
pressure  it  has  been  subjected  to  in  the  digestion  chamber  of  the 
Imhoff  tank. 

252.  Status  of  Imhoff  Tanks.— The  introduction  of  the 
Imhoff  tank  into  the  United  States,  like  the  introduction  of  the 
Burkli-Ziegler  Run-Off  Formula,  and  Kutter's  Formula,  is  to  be 
credited  to  Dr.  Rudolph  Hering.  He  advised  Dr.  Imhoff  to 
come  to  the  United  States  to  introduce  his  tank  and  gave  him 
material  aid  through  recommendations  and  introductions  to 
engineers.  Shortly  after  its  introduction,  in  1907,  the  tank 
became  very  popular  and  installations  were  made  in  many 
cities.  This  popularity  was  caused  by  a  growing  dissatisfaction 


426  SEPTICIZATION 

with  the  septic  tank,  the  litigation  then  progressing  over  septic 
patents,  the  production  of  inoffensive  sludge,  and  the  promising 
results  which  had  been  obtained  in  Germany.  As  a  result  of  the 
extended  experience  obtained  in  the  use  of  Imhoff  tanks  American 
engineers  have  learned  that,  like  all  other  sewage  treatment 
devices  introduced  up  to  the  present  time,  the  Imhoff  tank 
requires  experienced  attention  for  its  successful  operation.  These 
tanks  are  now  being  installed  in  the  place  of  septic  tanks,  and 
they  are  frequently  used  in  conjunction  with  sprinkling  filters. 

253.  Operation  of  Imhoff  Tanks. — The  important  feature 
in  the  successful  operation  of  an  Imhoflf  tank  is  the  proper 
control  of  the  sludge  and  transition  chambers.  During  the 
ripening  process,  which  may  occupy  2  weeks  to  3  months  after 
the  start  of  the  tank,  offensive  odors  may  be  given  off,  the  tank 
may  foam  violently,  and  scum  may  boil  over  into  the  sedi- 
mentation chamber.  This  is  usually  due  to  an  acid  condition 
in  the  digestion  chamber  which  may  possibly  be  overcome  by 
the  addition  of  lime.  A  very  fresh  influent  will  have  a  similar 
effect.  Too  violent  boiling  is  not  likely  to  occur  where  the 
area  for  the  escape  of  gas  has  been  made  large  and  the  gas  is 
not  confined.  Any  accumulation  of  scum  should  be  broken  up 
and  pushed  down  into  the  digestion  chamber,  or  removed  from 
the  tank.  The  stream  from  a  fire  hose  is  useful  in  breaking  up 
scum.  The  side  walls  of  the  sedimentation  chamber  should  be 
squeegeed  as  frequently  as  is  necessary  to  keep  them  free  from 
sludge,  which  may  be  as  often  as  once  or  twice  a  week.  Material 
floating  on  the  surface  of  the  sedimentation  chamber  should  be 
removed  from  the  tank  or  sunk  into  the  digestion  chamber 
through  the  gas  vents  in  the  transition  chamber. 

No  sludge  should  be  removed,  except  for  the  taking  of  samples, 
until  the  tank  is  well  ripened.  The  ripening  of  the  sludge  can  be 
determined  by  examining  a  sample  and  observing  its  color  and  odor. 
An  odorless,  black,  granular,  well  humified  sludge  is  indicative 
of  a  ripened  tank.  After  the  tank  has  ripened,  sludge  should  be 
removed  in  small  quantities  at  2  to  3-week  intervals,  except  in 
cold  or  rainy  weather.  The  sludge  should  be  drawn  off  slowly 
to  insure  the  removal  of  the  oldest  sludge  at  the  bottom  of  the 
digestion  chamber.  After  the  drawing  off  of  the  sludge  has 
ceased  the  pipe  should  be  flushed  with  fresh  water  to  prevent 
its  clogging  with  dried  sludge  in  the  interim  until  the  next 


OTHER  TANKS  427 

• 

removal.  Under  no  circumstances  should  all  the  sludge  in  the 
tank  be  removed  at  any  time.  The  removal  of  some  sludge 
during  foaming  after  ripening  may  reduce  or  stop  the  foaming. 
The  ripening  of  a  tank  can  be  hastened  by  adding  some  sludge 
from  a  tank  already  ripened. 

Sludge  should  not  be  allowed  to  accumulate  within  18  inches 
of  the  slot  at  the  bottom  of  the  digestion  chamber.  The  elevation 
of  the  surface  of  the  sludge  can  be  located  by  lowering  into  the 
tank,  a  stoppered,  wide-mouthed  bottle  on  the  end  of  a  stick. 
The  stopper  is  pulled  out  by  a  string  when  the  bottle  is  at  some 
known  elevation.  The  bottle  is  then  carefully  raised  and 
observed  for  the  presence  of  sludge.  The  process  is  repeated 
with  the  bottle  at  different  elevations  until  the  surface  of  the 
sludge  has  been  discovered.  Another  method  is  to  place  the 
suction  pipe  of  a  small  hand  pump  at  known  points,  successively 
increasing  in  depth,  and  to  pump  in  each  position  until  one  posi- 
tion is  found  at  which  sludge  appears  in  the  pump.  When  the 
sludge  in  one  portion  of  the  digestion  chamber  has  risen  higher 
than  in  another  portion,  the  direction  of  flow  in  the  sedimentation 
chamber  should  be  reversed  if  possible.  In  the  ordinary  routine 
of  operation  it  is  never  necessary  to  shut  down  an  Imhoff  tank. 
Sludge  is  removed  while  the  tank  is  operating.  The  shut  down 
of  a  tank  will  be  caused  by  accidents  and  breaks  to  the  structure 
or  control  devices. 

254.  Other  Tanks. — The  Travis  Hydrolytic  Tank  represents 
a  step  in  the  development  from  the  septic  tank  to  the  Imhoff 
tank.  The  Doten  tank  and  the  Alvord  tank  are  recent  develop- 
ments, and  are  somewhat  similar  in  operation  to  the  Imhoff 
tank. 

The  Travis  Hydrolytic  Tank  when  first  designed  differed 
from  the  later  design  of  the  Imhoff  tank  in  the  slot  between 
the  sedimentation  chamber  and  the  digestion  chamber  which 
was  not  trapped  against  the  escape  of  gas  from  the  latter  to  the 
former,  and  in  operation  a  small  quantity  of  fresh  sewage  was 
allowed  to  flow  through  the  digestion  chamber.  The  tank  is 
called  a  hydrolytic  tank  because  some  solids  are  liquefied  in  it. 
The  tank  is  mainly  of  historic  interest  as  designs  similar  to  it  are 
rarely  made  to-day.  Better  results  are  obtained  from  the  use 
of  the  Imhoff  tank.  Recent  developments  have  altered  the 
original  design  of  the  Travis  tank  so  that  it  is  hardly  recognizable. 


428 


SEPTICIZATION 


The  Travis  tank  at  Luton,  Eng.,  is  shown  in  Fig.  164.  The 
detailed  description  given  in  the  Engineering  News  in  connection 
with  this  illustration  shows  that  the  governing  object  of  the 
design  is  to  separate  as  quickly  as  possible  the  sludge  deposited 


HYDROLIZING 
CHAMBER-. 

**X 

J&- 


sl8"0utle+ Carrier 
•fo  Pilfer  Beds 
,9  "In  let 


FIG.  164. — Plan  and  Section  of  Hydrolytic  Tank  at  Luton,  England. 

Eng.  News,  Vol.  76,  1916,  p.  194. 

by  the  sewage  without  septic  action  being  set  up.  To  aid  in  the 
collection  and  settlement  of  flocculent  matter  vertical  wooden 
grids  or  colloiders  are  used.  The  suspended  matter  strikes  these 
and  forms  a  slimy  deposit  on  them  that  in  a  short  time  slips  off 
in  pieces  large  enough  to  settle  readily. 


OTHER  TANKS 


429 


The  Doten  tank  1  is  a  single-storied,  hopper-bottomed  septic 
tank,  views  of  which  are  shown  in  Fig.  165.  It  was  devised  by 
L.  S.  Doten  for  army  cantonments  during  the  War.  Its  chief 
purpose  was  to  avoid  the  foaming  and  frothing  so  common  to 
Imhoff  tanks  when  overdosed  with  fresh  sewage.  The  first 
Alvord  tank  was  constructed  in  Madison,  Wis.,  in  1913. 2  As 
now  constructed  the  tank  consists  of  three  deep,  single-story  com- 
partments with  hopper  bottoms.  These  compartments  are 
arranged  side  by  side  in  any  one  unit.  Sewage  enters  at  the  sur- 


<—     Slope  1.4% 


8  "C.I.  Soil  Pipe,  kludge  Drctfn^ 


K-  10  "Gatevalve       %<-8  "6ate  Valves - 


_£-• 4- 0*4-0  "trap  Doors       «, 
Slope  Gutter  to  End  of  Building 


Trap 
. — .  , — |  Doors 

-Q— 97  9  "~ tzr 

-  23-6-— *i,|c 23-6  -—>!,!< ?3- 

'V5-  '-/ 

K  -  8 "Gate  Valve  X, 


&heafh-,6*6" 
ing-, 


Plan  of  Septic  Tank 


Section  B-B 


FIG.  165. — Doten  Tank  for  Army  Cantonment  Sewage  Disposal. 

Eng.  News-Record,  Vol.  79,  1917,  p.  931. 


face  of  one  of  the  compartments  and  is  retained  here  during 
one-half  of  the  total  period  of  retention.  It  leaves  the  first 
compartment  over  a  weir  and  passes  in  a  channel  over  the  top 
of  the  intermediate  compartment  to  the  third  or  effluent  com- 
partment, where  it  is  held  for  the  remainder  of  the  period  of 
detention.  Accumulated  scum  and  sludge  are  drawn  off  into  the 
intermediate  compartment  at  the  will  of  the  operator,  this 

1  Trans.  Am.  Society  Civil  Engineers,  Vol.  83,  1920,  p.  337. 

2  Eng.  News  Record,  Vol.  83,  1919,  p.  510. 


430  SEPTICIZATION 

compartment  being  used  for  sludge  digestion  only.  Such  tanks 
as  the  Doten  and  the  Alvord  have  been  used  for  plants  receiving 
.very  fresh  sewages  such  as  is  discharged  from  military  canton- 
ments, in  order  to  assist  in  the  prevention  of  the  foaming  to  be 
expected  from  an  Imhoff  tank  receiving  such  a  fresh  influent. 
The  tanks  are  suitable  for  small  installations,  or  where  excavation 
to  the  depth  required  for  an  Imhoff  tank  is  not  practicable. 


CHAPTER  XVII 
FILTRATION   AND   IRRIGATION 

255.  Theory. — The  cycle  through  which  the  elements  forming 
organic  matter  pass  from  life  to  death  and  back  to  life  again 
has  been  described  in  Chapter  XIII.  It  has  been  shown  in 
Chapter  XVI  that  septic  action  occupies  that  portion  of  the 
cycle  in  which  the  combinations  of  these  elements  are  broken 
down  or  reduced  to  simpler  forms  and  the  lower  stages  of  the  cycle 
are  reached.  The  action  in  the  filtration  of  sewage  builds  up 
the  compounds  again  in  a  more  stable  form  and  almost  complete 
oxidation  is  attained,  dependent  on  the  thoroughness  of  the 
filtration.  In  the  filtration  of  sewage  only  the  coarsest  particles 
of  suspended  matter  are  removed  by  mechanical  straining.  The 
success  of  the  nitration  is  dependent  on  biologic  action.  The 
desirable  form  of  life  in  a  filter  is  the  so-called  nitrifying  bacteria 
which  live  in  the  interstices  of  the  filter  bed  and  feed  upon  the 
organic  matter  in  the  sewage.  Anything  which  injures  the 
growth  of  these  bacteria  injures  the  action  of  the  filter.  In  a 
properly  constructed  and  operated  filter,  all  matter  which  enters 
in  the  influent,  leaves  with  the  effluent,  but  in  a  different  molec- 
ular form.  A  slight  amount  may  be  lost  by  evaporation  and 
gasification  but  this  is  more  than  made  up  by  the  nitrogen  and 
oxygen  absorbed  from  the  atmosphere.  The  nitrifying  action 
in  sewage  filtration  is  shown  by  the  analysis  of  sewage  passing 
through  a  trickling  filter,  as  given  in  Tables  86  and  87.  It  is 
shown  by  the  reduction  of  the  content  of  organic  nitrogen,  free 
ammonia,  oxygen  consumed,  and  the  increase  in  nitrites,  nitrates, 
and  dissolved  oxygen.  The  reduction  of  suspended  matter  is 
interrupted  periodically  when  the  filter  "  unloads."  The  sus- 
pended matter  in  the  effluent  is  then  greater  than  in  the  influent. 

The  nitrifying  organisms  have  been  isolated  and  divided 
into  two  groups — nitrosomonas,  the  nitrite  formers,  and  nitrcbader, 
the  nitrate  formers.  Experiments  indicate  that  the  growth  of  the 

431 


432  FILTRATION  AND  IRRIGATION 

nitrobacter  organisms  is  dependent  on  the  presence  of  the 
nitrosomonas  organisms,  which  are  in  turn  dependent  on  the 
presence  of  the  putrefactive  compounds  resulting  from  the  action 
of  putrefying  bacteria.  The  existence  of  these  organisms  is  an 
example  of  symbiotic  action  in  bacterial  growth.  The  organisms 
have  been  found  to  grow  best  on  rough  porous  material  on  which 
their  zoogleal  jelly  can  be  easily  deposited  and  affixed.  Sewage 
filters  were  constructed  to  provide  these  ideal  conditions  before 
the  action  of  a  filter  was  thoroughly  understood. 

The  action  in  irrigation  is  similar  to  that  in  filtration. 
Although  more  strictly  a  method  of  final  disposal  rather  than 
preliminary  treatment,  the  similarity  of  the  actions  which  take 
place,  and  the  grading  of  sand  filtration  into  broad  irrigation 
with  no  distinct  line  of  difference  has  resulted  in  the  inclusion  of  the 
discussion  of  irrigation  in  the  same  chapter  with  filtration. 

256.  The  Contact  Bed. — A  contact  bed  is  a  water-tight  basin 
filled  with  coarse  material,  such  as  broken  stone,  with  which 
sewage  and  air  are  alternately  placed  in  contact  in  such  a  manner 
that  oxidation  of  the  sewage  is  effected.  A  contact  bed  has  some 
of  the  features  of  a  sedimentation  tank  and  an  oxidizing  filter. 
As  such  it  marks  a  transitory  step  from  anaerobic  to  aerobic 
.treatment  of  sewage.  A  plan  and  a  section  of  a  contact  bed  are 
shown  in  Fig.  166. 

Because  of  its  dependence  on  biologic  action  a  contact  bed 
must  be  ripened  before  a  good  effluent  can  be  obtained.  The 
ripening  or  maturing  occurs  progressively  during  the  first  few 
weeks  of  operation,  the  earlier  stages  being  more  rapidly 
developed.  The  time  required  to  reach  such  a  stage  of  maturity 
that  a  good  effluent  will  be  developed  will  vary  between  one  and 
six  or  eight  weeks,  dependent  on  the  weather  and  the  character 
of  the  influent.  During  the  period  of  maturing  the  load  on  the 
bed  should  be  made  light. 

The  use  of  contact  beds  has  been  extensive  where  a  more 
stable  effluent  than  could  be  obtained  from  tank  treatment  has 
been  desired,  yet  the  best  quality  of  effluent  was  not  required. 
The  sewage  to  undergo  treatment  in  a  contact  bed  should  be  given 
a  preliminary  treatment  to  remove  coarse  suspended  matter. 
The  efficiency  of  the  contact  treatment  can  be  increased  by 
passing  the  sewage  through  two  or  three  contact  beds  in  series. 
In  double  contact  treatment  the  primary  beds  are  filled  with 


THE  CONTACT  BED 


433 


coarser  material  and  operate  at  a  more  rapid  rate  than  the 
secondary  beds.  Double  contact  gives  better  results  than 
single  contact,  but  triple-contact  treatment,  though  showing 
excellent  results,  is  hardly  worth  the  extra  cost.  An  advantage 
which  contact  treatment  has  over  all  other  methods  of  sewage 
filtration  is  that  the  bed  can  be  so  operated  that  the  sewage  is 
never  exposed  to  view.  As  a  result  the  odors  from  well-operated 
contact  beds  are  slight  or  are  entirely  absent  and  there  should  be 


Sand          Filters 
151' 


Contact      ,     Beds 
148- 


Jior.  Scale. 
0     20'    40'    60' 


m.  A  A  I  \JopofWal\,  \EI.1123  \^-^  _1  __'"..   \ 

':%  I     Top  of  Wall,  \EIX73    p         4rh^lr"^^-'*^^- 

-*f9^PRWP  ••>  ""TopofSond.EI.3lM  ,t 


Longitudinal  Section. 

FIG.  166.— Plan  and  Section  of  Treatment  Plant  at  Marion,  Ohio,  Showing 

Septic  Tank,  Contact  Bed,  and  Sand  Filter. 

1908  Report  Ohio  State  Board  of  Health. 

no  trouble  from  flying  insects.  Such  a  method  of  treatment  is 
favorable  to  plants  located  in  populous  districts  and  to  the  fancies 
of  a  landscape  architect.  Another  advantage  of  the  contact 
bed  is  the  small  amount  of  head  required  for  its  operation, 
which  may  be  as  low  as  4  to  5  feet.  This  low  head  consumption 
by  a  sewage  filter  is  equaled  only  by  the  intermittent  sand 
filter. 

The  quality  of  the  effluent  from  some  contact  beds  is  shown 
in  Table  85.  It  is  to  be  noted  that  nitrification  has  been  carried 
to  a  fair  degree  of  completion,  and  that  the  reduction  of  oxygen 
consumed  has  been  marked.  In  comparison  with  the  effluent 


434 


FILTRATION  AND  IRRIGATION 


from  filters,  contact  effluent  contains  a  smaller  amount  of  nitro- 
gen as  nitrites  and  nitrates,  and  suspended  solids.  Contact 
effluent  is  usually  clear  and  odorless,  but  it  is  not  stable  without 
dilution.  The  absence  of  nitrites  and  nitrates  is  sometimes 
advantageous  as  the  effluent  will  not  support  vegetable  growths 
dependent  on  this  form  of  nitrogen.  The  absence  of  suspended 
solids  obviates  the  use  of  secondary  sedimentation  basins  which 
are  needed  with  trickling  'filters.  The  head  of  5  to  8  feet 
required  for  contact  treatment  is  law  in  comparison  to  the  10 
to  15  feet  required  for  trickling  filters,  but  is  slightly  higher  than 
the  head  required  for  intermittent  sand  filtration.  The  cost 
of  contact  treatment  is  higher  than  the  cost  of  trickling  filters 
but  is  lower  than  the  cost  of  intermittent  sand  filtration,  as 
shown  in  Table  90. 

TABLE  85 

QUALITY  OF  EFFLUENTS  FROM  CONTACT  BEDS 
Report  on  Sewage  Purification  at  Columbus,  Ohio,  by  G.  A.  Johnson,  1905. 


1 

Nitrogen  as 

Suspended 
Matter 

^i 

m 

Size  of 

Rate, 
Million 

I 

08 

I 

Material 

Gallons 

o 

'3 

T-I 

HH 

in  Inches 

per  Acre 

H 

55 

| 

a 

$ 

0) 

1 

h 

•g 

per  Day 

1 

'3 

Q 

Q 

£ 

OS 

3 

^ 

3 

a 

H 

8-S 

'C 

h 

3 

J5 

8 

• 

ra 

a) 
Q 

X 

O 

0 

1 

'1 

1 

6 

o 

i 

Q 

Parts  per  Million 

A 

5 

0.25-1.00 

0.953 

23 

3.5 

8.7 

0.20 

1.6 

832 

94 

737 

0.3 

B 

5 

0.25-2.00 

1.514 

21 

4.0 

8.4 

0.15 

1.4 

831 

85 

746 

0.1 

C 

5 

0.25-1.50 

1.222 

24 

3.5 

10.8 

0.11 

0.6 

826 

92 

734 

0.8 

D 

5 

0.50-1.50 

1.405 

22 

3.3 

9.5 

0.13 

0.9 

810 

91 

717 

0.9 

Per  Cent  Removal  of  Constituer 

its  of  Appli< 

;d  Sew 

age 

A 

5 

0.25-1.00 

0.953 

48 

49 

10 

73 

70 

76 

0  .  25-2  .  00 

1.514 

52 

40 

11 

80 

77 

83 

c 

5 

0.25-1.50 

1.222 

47 

31 

12 

70 

70 

70 

D 

5 

0.50-1.50 

1.405 

46 

37 

19 

67 

61 

72 

The  depth  of  the  contact  bed  is  generally  made  from  '4  to 
6  feet.  The  deeper  beds  are  less  expensive  per  unit  of  volume, 
to  construct,  as  the  cost  of  the  under  drains  and  the  distribution 
system  is  reduced  in  relation  to  the  capacity  of  the  filter.  The 
increased  depth  reduces  the  aeration,  and  the  periods  of  filling 


THE  CONTACT  BED  435 

and  emptying  are  so  increased  as  to  limit  the  depths  to  the  figures 
stated.  The  other  dimensions  of  the  bed  are  controlled  by 
economy  and  local  conditions,  as  the  success  of  the  contact 
treatment  is  not  affected  by  the  shape  of  the  bed.  Contact 
units  are  seldom  constructed  larger  than  one-half  an  acre  in  area, 
as  larger  beds  require  too  much  time  for  filling  and  emptying. 
A  large  number  of  small  units  is  also  undesirable  because  of  the 
increased  difficulty  of  control.  In  general  it  is  well  to  build  as 
large  units  as  are  compatible  with  efficient  operation,  elasticity 
of  plant,  and  which  can  be  filled  within  the  time  allowed  at  the 
average  rate  of  sewage  flow,  or  from  dosing  tanks  in  which  the 
storage  period  is  not  so  long  as  to  produce  septic  conditions. 

The  interstices  in  a  contact  bed  will  gradually  fill  up,  due  to 
the  deposition  of  solid  matter  on  the  contact  material,  the  dis- 
integration of  the  material,  and  the  presence  of  organic  growths. 
The  period  of  rest  allowed  every  five  or  six  weeks  tends  to  restore 
partially  some  of  this  lost  capacity  through  the  drying  of  the 
organic  growths.  It  is  occasionally  necessary  to  remove  the 
material  from  the  bed  and  wash  it  in  order  to  restore  the  original 
capacity.  It  may  be  necessary  to  do  this  three  or  four  times  a 
year,  in  an  overloaded  plant,  or  as  infrequently  as  once  in  five  or 
six  years  in  a  more  lightly  loaded  bed.  The  period  is  also 
dependent  on  the  character  of  the  contact  material  and  the  quality 
of  the  influent.  This  loss  of  capacity  may  reduce  the  voids  from  an 
original  amount  of  40  to  50  per  cent  of  voids  to  10  to  15  per 
cent.  If  the  bed  is  not  overloaded  the  loss  of  capacity  will  not 
increase  beyond  these  figures. 

The  rate  of  filtration  depends  on  the  strength  of  the  sewage, 
the  character  of  the  contact  material,  and  the  required  effluent. 
It  should  be  determined  for  any  particular  plant  as  the  result 
of  a  series  of  tests.  For  the  purposes  of  estimation  and  com- 
parison the  approximate  rate  of  filtration  should  be  taken  at 
about  94  gallons  per  cubic  yard  of  filtering  material  per  day  on 
the  basis  of  three  complete  fillings  and  emptyings  of  the  tank. 
This  is  equivalent  to  150,000  gallons  per  acre  foot  of  depth  per 
day,  or  for  a  bed  5  feet  deep  to  a  rate  of  750,000  gallons  per  acre 
per  day.  The  net  rate  for  double  or  triple  filtration  is  less  than 
these  figures,  but  on  each  filter  the  rates  are  higher. 

The  material  of  the  contact  bed  should  be  hard,  rough,  and 
angular.  It  should  be  as  fine  as  possible  without  causing  clogging 


436  FILTRATION  AND  IRRIGATION 

of  the  bed.  Materials  in  successful  use  are:  crushed  trap  rock 
or  other  hard  stone,  broken  bricks,  slag,  coal,  etc.  Soft  crumbling 
materials  such  as  coke  are  not  suitable  as  the  weight  of  the 
superimposed  material  and  the  movement  of  the  sewage  crushes 
and  breaks  it  into  fine  particles  which  accumulate  in  the  lower 
portion  of  the  filter  and  clog  it.  Roughness,  porosity,  and  small 
size  are  desirable,  as  the  greater  the  surface  area  the  more  rapid 
the  deposition  of  material.  After  a  short  time,  however,  the 
advantages  of  roughness  and  porosity  are  lost,  as  the  sediment 
soon  covers  all  unevenness  alike.  The  minimum  size  of  the 
material  is  limited  by  the  tendency  towards  clogging.  The  sizes 
in  successful  use  vary  between  J  and  j  of  an  inch,  J  inch  being 
a  common  size.  The  same  size  of  material  is  used  throughout 
the  depth  of  the  bed  except  that  the  upper  6  inches  may  be 
composed  of  small  white  pebbles  or  other  clean  material,  which 
does  not  come  in  contact  with  the  sewage  and  which  will  give 
an  attractive  appearance  to  the  plant.  In  double  or  triple  con- 
tact beds  3  or  4-inch  material  is  sometimes  used  for  the  primary 
beds,  and  J-inch  material  in  the  final  bed. 

Sewage  may  be  applied  at  any  point  on  or  below  the  surface. 
The  sewage  is  withdrawn  from  the  bottom  of  the  bed.  It  is 
undesirable  to  have  too  few  inlet  or  outlet  openings  as  the 
velocity  of  flow  about  the  openings  will  be  so  great  as  to  disturb 
the  deposit  on  the  contact  material.  The  distribution  system 
and  the  underdrains  for  the  bed  at  Marion,  Ohio,  are  shown  in 
Fig.  166. 

The  cycle  of  operation  of  a  contact  bed  is  divided  into  four 
periods.  A  representative  cycle  might  be:  time  of  filling,  one 
hour;  standing  full,  2  hours;  emptying,  one  hour;  standing 
empty,  4  hours.  The  length  of  these  periods  is  the  result  of  long 
experience  based  on  many  tests  and  are  an  average  of  the  conclu- 
sions reached.  Wide  variations  from  them  may  be  found  in 
different  plants,  and  tests  may  show  successful  results  with 
different  periods.  The  combination  of  these  four  periods  is  known 
as  the  contact  cycle. 

The  period  of  filling  should  be  made  as  short  as  possible 
without  disturbing  the  material  of  the  bed  nor  washing  off  the 
accumulated  deposits.  The  sewage  should  not  rise  more  rapidly 
than  one  vertical  foot  per  minute.  During  the  contact  or  stand- 
ing full  period  sedimentation  and  adsorption  of  the  colloids  are 


THE  TRICKLING  FILTER  437 

occurring  on  the  area  of  surface  exposed  to  the  sewage.  This 
period  should  be  of  such  length  that  septic  action  does  not  become 
pronounced,  and  long  enough  to  permit  of  thorough  sedimenta- 
tion. The  period  of  emptying  should  be  made  as  short  as  possible 
without  disturbing  the  bed,  on  the  same  basis  that  the  period 
of  filling  is  determined.  During  the  period  of  standing  empty, 
air  is  in  contact  with  the  sediment  deposited  in  thin  layers  on  the 
contact  material,  and  the  oxidizing  activities  of  the  filter  are  taking 
place.  The  filter  is  given  a  rest  period  of  one  or  two  days 
every  five  or  six  weeks,  in  order  that  it  may  increase  its 
capacity  and  it  biologic  activity. 

The  control  of  a  contact  bed  may  be  either  by  hand  or  auto- 
matic, the  latter  being  the  more  common.  Hand  control  requires 
the  constant  attention  of  an  operator  and  results  in  irregularity 
of  operation,  whereas  automatic  control  will  require  inspection 
not  more  than  once  a  day  and  insures  regularity  of  operation. 
A  number  of  automatic  devices  have  been  invented  which  give 
more  or  less  satisfaction.  The  air-locked  automatic  siphons, 
without  moving  parts,  have  proven  satisfactory  and  are  practi- 
cally "  fool-proof."  The  operation  of  these  devices  is  explained 
in  Chapter  XXI. 

257.  The  Trickling  Filter. — A  trickling  or  sprinkling  filter 
is  a  bed  of  coarse,  rough,  hard  material  over  which  sewage  is 
sprayed  or  otherwise  distributed  and  allowed  to  trickle  slowly 
through  the  filter  in  contact  with  the  atmosphere.  A  general 
view  of  a  trickling  filter  in  operation  at  Baltimore  is  shown  in 
Fig.  167.  The  action  of  the  trickling  filter  is  due  to  oxidation 
by  organisms  attached  to  the  material  of  the  filter.  The  solid 
organic  matter  of  the  sewage  deposited  on  the  surface  of  the 
material,  is  worked  over  and  oxidized  by  the  aerobic  bacteria, 
and  is  discharged  in  the  effluent  in  a  more  highly  nitrified  con- 
dition. At  times  the  discharge  of  suspended  matter  becomes 
so  great  that  the  filter  is  said  to  be  unloading.  The  action  differs 
from  that  in  a  contact  bed  in  that  there  is  no  period  of  septic 
or  anaerobic  action  and  the  filter  never  stands  full  of  sewage. 

The  effluent  from  a  trickling  filter  is  dark,  odorless,  and  is 
ordinarily  non-put rescible.  Analyses  of  typical  effluents  are 
given  in  Tables  86  and  87.  The  unloading  of  the  filter  may  occur 
at  any  time,  but  is  most  likely  to  occur  in  the  spring  or  in  a 
warm  period  following  a  period  of  low  temperatures.  It  causes 


438 


FILTRATION  AND  IRRIGATION 


higher  suspended  matter  in  the  effluent  than  in  the  influent 
and  may  render  the  effluent  putrescible.  The  action  is  marked 
by  the  discharge  of  solid  matter  which  has  sloughed  off  of  the 
filter  material  and  which  increases  the  turbidity  of  the  effluent. 
Where  the  diluting  water  is  insufficient  to  care  for  the  solids  so 
carried  in  the  effluent,  they  can  be  removed  by  a  2-hour  period 
of  sedimentation.  The  effluent  may  become  septic  during  this 
time,  however.  The  nitrogen  in  the  effluent  is  almost  entirely 
in  the  form  of  nitrates,  and  the  percentage  of  saturation  with 
dissolved  oxygen  is  high.  The  effluent  is  more  highly  nitrified 
than  that  from  a  contact  bed,  and  its  relative  stability  is  also 
higher,  thus  demanding  a  smaller  volume  of  diluting  water. 


FIG,  167. — Sprinkling  Filter  in  Operation  in  Winter  at  Baltimore. 

The  principal  advantage  of  a  trickling  filter  over  other  methods 
of  treatment  is  its  high  rate  which  is  from  two  to  four  times  faster 
than  a  contact  bed,  and  about  seventy  times  faster  than  an  inter- 
mittent sand  filter.  The  greatest  disadvantage  is  the  head  of  12  to 
15  feet  or  more  necessary  for  its  operation.  Sedimentation  of  the 
effluent  is  usually  necessary  to  remove  the  settleable  solids. 
During  the  period  of  secondary  sedimentation  the  quality  of  the 
filter  effluent  may  deteriorate  in  relative  stability.  In  winter  the 
formation  of  ice  on  the  filter  results  in  an  effluent  of  inferior 
quality,  but  as  the  diluting  water  can  care  for  such  an  effluent 
at  this  time  the  condition  is  not  detrimental  to  the  use  of  the 
trickling  filter.  In  summer  the  filters  sometimes  give  off  offen- 
sive odors  that  can  be  noticed  at  a  distance  of  half  a  mile,  and 
flying  insects  may  breed  in  the  filter  in  sufficient  quantities  to 


THE  TRICKLING  FILTER 


439 


become  a  nuisance  if  preventive  steps  are  not  taken.  The  dis- 
semination of  odors  is  especially  marked  when  treating  a  stale 
or  septic  sewage.  The  treatment  of  a  fresh  sewage  seldom  results 
in  the  creation  of  offensive  odors. 

TABLE  86 

ANALYSIS  OF  CRUDE  SEWAGE,  IMHOFF  TANK,  AND  SPRINKLING  FILTER 
EFFLUENTS  AT  ATLANTA,  GEORGIA 

(Engineering  Record,  Vol.  72,  p.  4) 


Temperature 
Fahrenheit 

Parts  per  Million 

Cent  Saturation, 
dissolved  Oxygen 

ative  Stability 

Nitrogen  as 

Oxygen  Con- 
sumed 

Suspended  Matter 

Organic 

Free  Am- 
monia 

| 

Nitrates 

"(3 

.2 

eS 

1 

1 

£ 

Crude  Se-mage 


1913 

Maximum  

77 

15.6 

21.8 

0.1 

3.0 

100.0 

371 

154 

163 

47 

Minimum  

61 

10.4 

16.5 

0.1 

1.4 

78.3 

222 

98 

112 

11 

Average  

70 

12.8 

18.8 

0.1 

2.2 

90.6 

285 

126 

138 

28 

1914  (7  months) 

Maximum  .... 

74 

16.9 

33.4 

2.3 

431 

48 

.Minimum          . 

60 

9.5 

18.1 

1.6 

279 

12 

Average  

66 

13.4 

27.1 

2.0 

351 

30 

Imhoff  Effluent 


1913 

Maximum  

78 

13.2 

21.9 

0.2 

3.1 

68.0 

90 

50 

41 

Minimum  

58 

6.5 

16.8 

0.1 

1.1 

53.1 

35 

42 

21 

Average  

68 

9.0 

20.0 

0.2 

2.1 

60.1 

68 

46 

33 

1914  (7  months) 

Maximum  ..... 

77 

10.3 

30.3 

2.0 

73 

48 

Minimum  

59 

4.1 

18.0 

1.5 

49 

34 

Average  

65 

7.7 

25.9 

1.8 

65 

43 

Sprinkling  Filter  Effluent 


1913 

Maximum  

79 

5.6 

14.2 

0.8 

11.3 

32.1 

60 

31 

28 

76 

99 

Minimum  

55 

2.6 

6.2 

0.5 

5.8 

23.6 

33 

26 

28 

52 

88 

Average  

66 

3.8 

9.9 

0.7 

8.2 

28.2 

49 

28 

28 

64 

89 

1914  (7  months) 

Maximum  

77 

8.5 

20.7 

11.2 

106 

79 

99 

Minimum  

55 

4.4 

8.8 

3.6 

40 

55 

89 

Average  

63 

5.7 

15.2 

7.2 

62 

65 

95 

440 


FILTRATION  AND  IRRIGATION 


3 


9    I 
S    | 


Oxygen 
Consumed 


Organic 
Nitrogen 


UOT11TJA[ 


OOO^- 

CDlClt- 


i-l    O    Tji 

CQ  eo  cc  t^  8   8 


J8d  S^JBJ 


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paAOuia'jj 


uoiniP\[  Jad  s^J1Bd[ 


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t^  000305COOOOO 

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•*  Tti.co  cc 


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o    O    o 

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c 

Illlll! 


THE  TRICKLING  FILTER  441 

Raw  sewage  cannot  be  treated  successfully  on  a  trickling 
filter.  Coarse  solid  particles  should  be  screened  and  settled  out, 
in  order  that  the  distributing  devices  or  the  filter  may  not  become 
clogged.  The  effluent  from  an  Imhoff  tank  has  proven  to  be  a 
satisfactory  influent  for  a  trickling  filter.  A  septic  tank  effluent 
may  be  so  stale  as  to  be  detrimental  to  the  biologic  action  in  the 
filter. 

In  the  operation  of  a  trickling  filter  the  sewage  is  sprayed 
or  otherwise  distributed  as  evenly  as  possible  in  a  fine  spray  or 
stream,  over  the  top  of  the  filtering  material.  The  sewage  then 
trickles  slowly  through  the  filter  to  the  underdrains  through 
which  it  passes  to  the  final  outlet.  The  distribution  of  the 
sewage  on  the  bed  is  intermittent  in  order  to  allow  air  to  enter 
the  filter  with  the  sewage.  The  cycle  of  operation  should  be 
completed  in  5  to  15  minutes,  with  approximately  equal  periods 
of  rest  and  distribution.  Cycles  of  too  great  length  will  expose 
the  filter  to  drying  or  freezing  and  will  give  poorer  distribution 
throughout  the  filter.  Cycles  which  are  too  short  will  operate 
successfully  only  with  but  slight  variation  in  the  rate  of  sewage 
flow.  In  some  plants  it  has  been  found  advantageous  to  allow 
the  filters  to  rest  for  one  day  in  3  to  6  weeks  or  longer,  dependent 
on  the  quality  of  the  effluent. 

The  rate  of  filtration  may  be  as  high  as  2,000,000  gallons  per 
acre  per  day,  which  is  equivalent  to  200  gallons  per  cubic  yard 
of  material  per  day  in  a  bed  6  feet  deep.  This  is  more  than 
double  the  rate  permissible  in  a  contact  bed.  The  exact  rate 
to  be  used  for  any  particular  plant  should  be  determined  by 
tests.  It  is  dependent  on  the  quality  of  the  sewage  to  be  treated, 
on  the  depth  of  the  bed,  the  size  of  the  filling  material,  the 
weather,  and  other  minor  factors. 

The  filtering  material  is  similar  to  that  used  in  a  contact  bed. 
It  should  consist  of  hard,  rough,  angular  material,  about  1  to 
2  inches  in  size.  Larger  sizes  will  permit  more  rapid  rates  of 
filtration,  but  will  not  produce  so  good  an  effluent.  Smaller 
sizes  will  clog  too  rapidly. 

The  depth  of  the  filter  is  limited  by  the  possibility  of  ventila- 
tion and  the  strength  of  the  filtering  material  to  withstand  crush- 
ing. The  deeper  the  bed  the  less  the  expense  of  the  distribution 
and  collecting  system  for  the  same  volume  of  material,  and  the 
more  rapid  the  permissible  rate  of  filtration.  •  The  depths  in 


442  FILTRATION  AND  IRRIGATION 

use  vary  between  6  and  10  feet,  with  6  to  8  feet  as  a  satisfactory 
mean.  From  a  biologic  standpoint  the  action  of  the  filter  seems 
to  be  proportional  to  the  volume  of  the  filtering  material  and 
therefore  proportional  to  the  depth  of  the  bed,  being  limited  to 
a  minimum  depth  of  about  5  feet,  below  which  sewage  may  pass 
through  the  filter  without  treatment.  The  shape  and  other 
dimensions  of  the  filter  depend  on  the  local  conditions  and  the 
economy  of  construction.  The  filters  need  not  be  broken  up 
into  units  by  water-tight  dividing  walls.  One  filter  can  be 
constructed  sufficient  for  all  needs  and  various  portions  of  it  can 
be  isolated  as  units  by  the  manipulation  of  valves  in  the  dis- 
tribution system.  Ventilation  is  provided  by  the  air  entrained 
with  the  sewage  as  it  falls  upon  the  surface.  If  the  sides  of  the 
filter  are  built  of  open  stone  crib  work  the  ventilation  will  be 
greatly  improved,  but  it  will  not  be  possible  to  flood  the  filters 
to  keep  down  flies,  and  in  cold  climates  these  openings  must  be 
covered  in  winter  to  prevent  freezing.  Filters  have  been  con- 
structed without  side  walls,  the  filtering  material  being  allowed 
to  assume  its  natural  angle  of  repose.  This  has  usually  been 
found  to  be  more  expensive  than  the  construction  of  side  retaining 
walls,  due  to  the  unused  filling  material  and  the  extra  under- 
drains  required. 

The  distribution  of  sewage  is  ordinarily  effected  by  a  system 
of  pipes  and  spray  nozzles  as  shown  in  Fig.  168  and  169.  Other 
methods  of  distribution  have  been  used.  At  Springfield,  Mo.,1 
a  moving  trough  from  which  the  sewage  flows  continuously  is 
drawn  back  and  forth  across  the  bed  by  means  of  a  cable.  In 
England  circular  beds  have  been  constructed  and  the  sewage 
distributed  on  them  through  revolving  perforated  pipes.  At 
the  Great  Lakes  Naval  Training  Station  2  the  distributing  pipes 
in  the  plant,  now  abandoned,  were  supported  above  the  surface 
of  the  filter.  The  sewage  fell  from  holes  in  the  lower  side  of  these 
pipes  on  to  brass  splash  plates  14  inches  above  the  filter.  It 
was  deflected  horizontally  from  these  plates  over  the  filter  sur- 
face. Pipes  and  spray  nozzles  have  been  adopted  almost  univer- 
sally in  the  United  States.  Splash  plates,  traveling  distributors, 
and  other  forms  of  distribution  have  been  used  only  in  excep- 

!See  Eng.  News,  Vol.  70,  1913,   p.  1112;   Eng.  Record,  Vol.  68,  1913,  p. 
440,  and  Eng.  News,  Vol.  75,  1916,  p.  1028. 
2  See  Eng.  Record,  Vol.  67,  1913,  p.  232. 


THE   TRICKLING  FILTER 


443 


tional  cases.  In  a  distributing  system  consisting  of  pipes  and 
nozzles,  a  network  of  pipes  is  laid  out  somewhat  as  shown  in 
Fig.  168,  in  such  a  manner  that  the  head  loss  to  all  points  is 
approximately  equal.  The  number  of  valves  required  should 
be  reduced  to  a  minimum.  The  pipes  may  be  laid  out  with  the 
main  feeders  leading  from  a  central  point  and  branches  at  right 
angles  to  them,  somewhat  on  the  order  of  a  spider's  web,  or  they 
may  be  laid  out  on  a  rectangular  or  gridiron  system.  The 
radial  system  is  advantageous  because  of  the  central  location 


^Crushed  Sfone  Filling  I  "fo  2 
El.364.li':'' 


Cross  Section  A-A. 

Grade  0.00?5 


21  "Side  Drain 


« 

§ 

-- 

fc     :           i 

rf/2  "C.  I.  Pipe                               j 

•a? 

,     ^ 

^ 

I            V. 

ji--« 

!             CM         Nozz/es^ 

!6'yi2"Reducer-' 

> 

v'Flushmq  Gallery 

£= 

:               i 

1 

6  "C.  i.  Wafer  Mam^  ^  6  "Disch.  Pipe 

Grade  0.0028''  ;    ^30"By-Hass  30  Concrete 

Drain  Conduit-™ 

Plan 

FIG.  168. — Section  through  Sprinkling  Filter  at   Fitchburg,  Mass.,  Showing 
Distribution  System. 

Eng.  Record,  Vol.  67,  p.  634. 

of  the  control  house,  but  it  does  not  always  lend  itself  favorably 
'to  the  local  conditions,  and  the  piping  and  nozzle  location  are 
not  so  simple.  The  gridiron  system  lends  itself  favorably  to 
the  equalization  of  head  losses.  The  pipes  used  should  be  larger 
than  would  be  demanded  by  considerations  of  economy  alone, 
both  for  the  purpose  of  reduction  of  head  loss  and  ease  in  cleaning. 
No  pipe  less  than  6  inches  in  diameter  should  be  used,  and  the 
average  velocity  of  flow  should  not  exceed  one  foot  per  second. 
Cast  iron,  concrete,  or  vitrified  clay  pipe  may  be  used,  but  cast 


444  FILTRATION  AND  IRRIGATION 

iron  is  the  material  commonly  used.  The  system  should  be 
arranged  for  easy  flushing  and  cleaning  and  the  pipes  so  sloped 
that  the  entire  system  can  be  drained  in  case  of  a  shut  down 
in  cold  weather. 

The  pipes  are  placed  far  enough  below  the  surface  of  the 
filling  material  so  that  the  top  of  the  spraying  nozzle  is  6  to 
12  inches  above  the  surface  of  the  filter.  If  the  pipes  are  placed 
near  the  surface  they  are  accessible  for  repairs,  but  are  exposed 
to  temperature  changes.  If  the  pipes  are  large  their  presence 
near  the  surface  of  the  filter  may  seriously  affect  the  distribution 
of  the  sewage  through  the  filter.  If  the  distributing  pipes  are 
placed  near  the  bottom  of  the  filter  they  are  inaccessible  for 
repairs  and  the  nozzles  must  be  connected  to  them  by  means 
of  long  riser  pipes.  The  distributing  pipes  should  be  supported 
by  columns  extending  to  the  foundation  of  the  filter  bed,  there 
being  a  column  at  every  pipe  joint  with  such  intermediate  sup- 
ports as  may  be  required.  In  some  plants  the  pipes  have  been 
supported  by  the  filtering  material.  Although  slightly  less 
expensive  in  first  cost  the  practice  of  so  supporting  the  pipes  is 
poor,  as  settling  of  the  material  may  break  the  pipe  or  cause 
leaks,  and  if  the  bed  becomes  clogged,  removal  of  the  material 
is  made  more  difficult.  Valves  should  be  placed  in  the  distribut- 
ing system  in  such  a  manner  that  different  sets  of  nozzles  can 
be  cut  out  at  will,  thus  resting  those  portions  of  the  filter 
and  permitting  repairs  without  shutting  down  the  entire 
filter. 

The  spacing  of  the  nozzles  is  fixed  by  the  type  and  size  of  the 
nozzle,  the  available  head,  and  the  rate  of  filtration.  Various 
types  of  sprinkler  nozzles  are  shown  in  Fig.  169  and  the  dis- 
charge rates,  head  losses,  and  distances  to  which  sewage  is 
thrown  for  the  Taylor  nozzles,  are  shown  in  Fig.  170.  Nozzles 
are  available  which  will  throw  circular,  square,  or  semicircular 
sprays.  In  the  use  of  circular  sprays  there  is  necessarily  some 
portion  of  the  filter  which  is  underdosed  if  the  nozzles  are  placed 
at  the  corners  of  squares  with  the  sprays  tangent,  and  there  is 
an  overdosing  of  other  portions  if  the  sprays  are  allowed  to 
overlap  so  that  no  portion  of  the  filter  is  left  without  a  dose. 
Rectangular  sprays  will  apparently  overcome  these  difficulties, 
but  studies  have  shown  that  circular  sprays  with  some  over- 
lapping, and  the  nozzles  placed  at  the  apexes  of  equilateral  tri- 


THE  TRICKLING  FILTER 


445 


i. 


hase 
Square 
Nozzle 


A= Diameter 
of  Orifice. 

Diameter 
of  Spindle 
at  Orifice. 


Priestman-Beddoes         Weand,  Atlantic  Type 
Round  Nozzle.  Round  Nozzle.  Worcester'   Round 

FIG.  169.— Sprinkling  Filter  Nozzles. 

Bulletin  No.  3,  Engineering  Experiment  Station,  Purdue  University. 

Discharge,   Gallons    per  Min-ute 
30       28         26        24        22        20        18         16         14         12         10 


10 


12  14  16 

Nozzle    Spacing,  Feet., 


20 


FIG.  170.— Diagram  Showing  the  Discharge  and  Spacing  of  Taylor  Nozzles. 


446 


FILTRATION  AND  IRRIGATION 


angles  as  shown  in  Fig.  172  will  give  as  satisfactory  distribution 
as  other  forms. 

The  nozzles  should  be  selected  to  give  the  best  distribution, 
to  consume  all  of  the  head  available,  and  to  give  the  proper 
cycle  of  operation.  The  entire  head  available  should  be  consumed 
in  order  that  the  fewest  number  of  nozzles  may  be  used.  An 
excellent  study  of  the  characteristics  of  various  types  of  nozzles 
has  been  published  in  Bulletin  No.  3  of  the  Engineering  Experi- 
ment Station  at  Purdue  University,  1920.  As  a  result  of  the 
tests  on  the  nozzles  shown  in  Fig.  169,  it  was  determined  for  all 
nozzles,  except  No.  8,  that 


in  which  Q  =the  rate  of  discharge  in  cubic  feet  per  second; 
C  =a  coefficient  shown  in  Table  88; 
a=the   net   cross-sectional    opening    of   the   nozzle  in 

square  feet; 
h  =the  pressure  on  the  nozzle  in  feet  of  water. 

TABLE  88 
COEFFICIENTS  OF  DISCHARGE  FOR  SPRINKLER  NOZZLES  SHOWN  IN  FIG.  169 


Nozzle  Number 

1 

2 

3 

4 

5 

6 

7 

Coefficient         

648 

756 

696 

666 

675 

598 

569 

It  is  evident  that  if  the  head  on  the  nozzles  is  constant  and  the 
nozzle  throws  a  circular  spray,  the  intensity  of  dosing  at  the 
circumference  will  be  greater  than  nearer  the  center.  This 
difficulty  is  overcome  by  so  designing  the  dosing  tank  from  which 
the  sewage  is  fed  that  the  head  on  the  nozzle  and  the  quantity 
thrown  will  vary  in  such  a  manner  that  the  distribution  over 
the  bed  is  equalized.  Intermittent  action  is  obtained  by  an 
automatic  siphon  which  commences  to  discharge  when  the  tank 
is  full  and  empties  the  tank  in  the  period  allowed  for  dosing. 
Under  such  conditions  the  tank  should  discharge  for  a  longer 
time  at  the  higher  heads  than  at  the  lower  heads  as  there  is 
more  territory  to  be  covered  at  the  higher  heads.  The  design 
of  the  tank  to  do  this  with  exactness  is  difficult,  and  the  con- 
struction of  the  necessary  curved  surfaces  is  expensive.  Where 


THE  TRICKLING  FILTER 


447 


a  dosing  tank  is  used  for  such  conditions  it  has  been  found  satis- 
factory to  construct  the  tank  with  plane  sides  sloping  at  approxi- 
mately 45  degrees  from  the  vertical  (or  horizontal).  A  tank 
with  curved  surfaces  is  shown  in  Fig.  171.  The  dosing  siphon 
is  usually  placed  in  the  tank  as  shown  in  the  figure.  The  head 
and  quantity  of  discharge  through  the  nozzles  can  be  varied 
also  by  maintaining  a  constant  depth  in  a  dosing  tank  by  means 


<-iz- 


f> 

•% 

•/>-*> 


V4 

•7>a:.\>»".: 


3'-v/  • 

Elevation  of  Sprinkler  Nozzle 


FIG.  171.  —  Section  of  12-inch  Siphon  and  Dosing  Tank,  for  King's  Park, 

Island. 


Long 


of  a  float  feed  valve,  and  varying  the  head  and  quantity  dis- 
charged to  the  nozzles  by  a  butterfly  valve  in  the  main  feed  line, 
or  by  the  use  of  a  Taylor  undulating  valve  designed  for  this 
purpose.  The  butterfly  valve  is  opened  and  closed  by  a  cam 
so  designed  and  driven  at  such  a  rate  that  the  required  distribu- 
tion is  obtained.  The  Taylor  undulating  valve  is  opened  and 
closed  at  a  constant  rate,  the  shape  of  the  valve  giving  the 
required  variations  in  head  and  discharge  Other  methods 
of  control  have  been  attempted  but  have  not  been  used  exten- 
sively. 

An  example  of  the  design  of  the  nozzle  layout  and  dosing 
tank  for  a  sprinkling  filter  follows  : 


448  FILTRATION  AND  IRRIGATION 

Let  it  be  required  to  determine  the  nozzle  layout 
for  one  acre  of  sprinkling  filters  with  5  feet  available  head 
on  the  nozzles. 

The  selection  of  the  type  of  nozzle  and  the  size  of 
opening  is  a  matter  of  judgment  and  experience.  Noz- 
zles with  large  openings  are  less  liable  to  clog  and  fewer 
nozzles  are  needed  than  where  small  nozzles  are  used, 
but  the  distribution  of  sewage  is  not  so  even  as  with  the 
use  of  small  nozzles.  In  this  example  Taylor  circular 
spray  nozzles  will  be  selected.  Fig.  170  shows  that  a 
Taylor  circular  spray  nozzle  will  discharge  22.3  g.p.m. 
under  a  head  of  5  feet,  and  that  the  economical  nozzle 
spacing  will  be  15.3  feet.  The  least  number  of  nozzles 
at  this  spacing  required  for  a  bed  of  one  acre  in  area  is 
found  as  follows:  In  Fig.  172,  let  n  equal  the  number  of 


FIG.  172. — Typical  Sprinkler  Nozzle  Layout. 

nozzles  in  a  horizontal  row,  counting  half -spray  nozzles  as  J, 
and  let  ra  equal  the  number  of  rows  counting  rows  of  half- 
spray  nozzles  as  half  rows.1  Then  the  number  of  nozzles, 
N,  equals  mn,  and  15.3wXl3.2n  equals  43,560  or  mn 
equals  215. 

The  next  step  should  be  the  design  of  the  dosing  tank  and 
siphon.  It  is  possible  to  design  a  tank  which  will  give  equal 
distribution  over  equal  areas  of  filter  surface.  It  has  been 

1  The  use  of  half-spray  nozzles  is  not  always  advocated  as  it  is  consid- 
ered that  their  use  does  not  markedly  improve  the  distribution.  Where  half 
nozzles  are  used,  a  margin  of  18  inches  to  2  feet  should  be  allowed  between 
the  edge  of  the  filter  and  the  nozzle,  to  prevent  the  blowing  of  raw  sewage 
from  the  filter. 


THE  TRICKLING  FILTER  449 

found,  however,  that  the  expense  of  this  refinement  is  unwar- 
ranted as  there  are  a  number  of  outside  factors  which  tend  to 
overcome  the  theoretical  design.  The  effect  of  wind,  unequal 
spacing,  and  irregularities  in  the  elevation  of  the  nozzles  have  a 
tendency  to  offset  refinements  in  the  design  of  a  dosing  tank. 
It  is  therefore  the  general  practice  to  slope  the  sides  of  the  tank 
at  an  angle  of  about  45  degrees  as  previously  stated.  The 
dosing  tank  is  generally  designed  to  have  a  capacity  which  will 
give  a  complete  cycle  of  operation  once  in  15  minutes.  In  the 
ordinary  design  the  factors  given  are  the  rate  of  inflow  and  the 
given  time  of  filling.  In  the  following  example  the  time  of  filling 
will  be  taken  as  10  minutes,  the  time  of  emptying  as  5  minutes, 
and  the  rate  of  flow  as  1,000,000  gallons  per  day.  The  capacity 

1  000  000 
of   the   tank   will   therefore   be      '       '        =7,000   gallons.     The 

diameter  of  the  siphon  to  be  selected  can  be  computed  as  follows  : 

Let  Q  =the  capacity  of  the  tank  in  cubic  feet; 

qi  =  the  rate  of  discharge  of  the  siphon  in  cubic  feet  per  second  ; 
#2  =  the  rate  of  inflow  to  the  tank  in  cubic  feet  per  second  ; 
q  =  the  rate  of  emptying  the  tank  in  cubic  feet  per  second  = 

(tfi-22); 
A  =the  cross-sectional  area  of  the  free  surface  of  the  water 

in  the  tank  at  any  instant,  in  square  feet; 
a  =  the  cross-sectional  area  of  the  siphon  in  square  feet  ; 
b  =  the  small  dimension  of  the  base  of  the  tank  in  feet  ; 
h  =the  head  of  water,  in  feet,  on  the  discharge  siphon; 
hi  =the  initial  head  of  water,  in  feet,  on  the  siphon; 
/i2  =  the  final  head  of  water  in  feet,  on  the  siphon  ; 
t  =the  time,  in  seconds,  required  to  empty  the  tank, 

then  dQ  =  —Adh=qidt—  q2dt, 

dQ     -Adh 

and  dt=-  -  =  —     —  , 

q      qi-q* 

but  qi=QAAV2Jjh,1 

Ch*       -Adh 
therefore  t= 

but  A  =4/i2+46/i+&2, 

therefore  t= 


QAaV2gh-q2 
From  paper  by  E.  G.  Bradbury  in  Proceedings  of  the  Ohio  Eng.  Society, 


1910,  p.  79. 


450 


FILTRATION  AND   IRRIGATION 


The  integration  of  this  expression  is  tedious.  Its  solution 
for  siphons  between  6  inches  and  12  inches  operating  under 
heads  commencing  from  3  feet  to  6  feet,  with  a  time  of  emptying 
of  5  minutes  and  time  of  filling  of  10  minutes  is  given  in  Fig. 
173.  In  the  example  given  the  rate  of  inflow  is  1.55  sec.  feet 
and  the  head  is  5  feet.  Then  from  Fig.  173  the  size  of  the  siphon 
to  be  used  is  12  inches.  Where  a  siphon  of  the  size  required 

Rate  of  Inflow;  Cubic  Feet  per  Second 
0.3    0.4     0.5     0.6      0.7     0.8     0.9      1.0      1.1       1.2     1.3     1.4      1.5      1.6     .1.7 


if 


2000 


7000 


8000 


3000  4000  5000  6000 

Capacity  of  Tank  in  Gallons. 

FIG.  173. — Diagram  for  the  Determination  of  the  Capacities  of  Dosing  Tanks 

for  Trickling  Filters. 

Time  of  emptying,  5  minutes.  Time  of  filling,  10  minutes.  Shape  of  tank  is  a  right  pyramid 
or  a  truncated  right  pyramid  with  all  four  sides  making  an  angle  of  45  degrees  with  the  ver- 
tical. All  horizontal  cross-sections  are  squares. 

to  empty  the  tank  in  the  time  fixed  is  not  available,  combinations 
of  available  sizes  can  sometimes  be  used. 

For  example,  if  the  given  head  is  6  feet,  and  the  rate 
of  inflow  is  1.4  sec.  feet,  it  is  evident  from  Fig.  173  that 
a  6,300-gallon  dosing  tank  and  two  8-inch  siphons  will 
give  the  required  cycle. 

The  method  used  for  the  design  of  the  setting  of  Taylor 
nozzles  by  the  Pacific  Flush  Tank  Co.,  is  less  rational  but  more 
simple  and  probably  as  satisfactory.  In  this  method  the  steps 
are  as  follows: 

(1)  Divide  the  maximum  daily  rate  of  sewage  flow  by 
1,000  to  get  the  maximum  minute  inflow. 


THE  TRICKLING  FILTER  451 

(2)  The  number  of  nozzles  required  is  determined  by 
dividing  the  preceding  figure  by  6.     Generally  a  Taylor 
nozzle  with  an  orifice  of  f  of  an  inch  will  discharge  about 
20  g.p.m.  at  the  high  head  and  about  8  g.p.m.  at  the  low 
head,   and  as  the   nozzles  must   have  a   capacity   which 
will  take  care  of  the  inflow  at  the  low  head,  the  divisor  6 
is  used  as  a  factor  of  safety  instead  of  using  8  as  the 
divisor. 

(3)  The  type  of  nozzle  to  be  used  is  selected  from 
experience  or  as  a  matter  of  judgment.     Circular-spray 
nozzles  are  more  generally  used. 

(4)  The  spacings  are  determined  from  Fig.  170. 

(5)  The  dosing  tank  of  the  shape  described  is  then 
designed.     The  capacity  is  such  as  to  give  a  complete 
cycle  once  every  15  minutes.     The  method  of  this  design 
is  similar  to  that  followed  previously. 

(6)  The  dosing  siphons  are  designed  so  that  they  will 
have  a  capacity  at  the  minimum  head  of  from  40  to  50 
per  cent  in  excess  of  the  maximum  minute  inflow,  and  the 
draining  depth  of  the  siphon  will  be  limited  to  a  maximum 
of  5  to  5J  feet.     The  siphons  are  all  made  adjustable  with 
a  variation  of  6  inches  or  more  on  either  side  of  the  normal 
discharge  line  so  that  the  spraying  area  and  cycle  can  be 
varied  to  secure  the  best  results. 

The  underdrainage  of  a  trickling  filter  should  consist  of  some 
form  of  false  bottom  such  as  the  types  shown  in  Fig.  174.  Where 
possible  the  underdrains  should  be  open  at  both  ends  for  the 
purpose  of  ventilation  and  flushing.  It  is  desirable  that  the 
drains  be  so  arranged  that  a  light  can  be  seen  through  them  in 
order  that  clogging  can  be  easily  located.  The  drains  should  be 
placed  on  a  slope  of  approximately  2  in  100  towards  a  main 
collector.  The  length  of  the  drains  is  limited  by  their  capacity 
to  carry  the  average  dose  from  the  area  drained  by  them. 
The  main  collecting  conduits  must  be  designed  in  accordance 
with  the  hydraulic  principles  given  in  Chapter  IV.  No  valves, 
or  other  controlling  apparatus,  are  placed  on  the  underdrains 
or  outlets  from  the  filter. 

Covers  have  been  provided  in  winter  for  some  trickling 
filters  in  cold  climates.  The  Taylor  sprinkling  nozzle  has  been 
found  to  work  successfully  in  extremely  cold  weather,  and  it  is 
generally  accepted  that  the  covering  of  filters  is  unnecessary,  if 
the  filter  is  not  to  be  shut  down  for  any  length  of  time  in  cold 
weather. 


452 


FILTRATION  AND  IRRIGATION 


The  operation  of  devices  for  automatically  controlling  the 
operation  of  a  trickling  filter  is  explained  in  Chapter  XXI. 

258.  Intermittent  Sand  Filter. — An  intermittent  sand  filter 
is  a  specially  prepared  bed  of  sand,  or  other  fine  grained  material, 
on  the  surface  of  which  sewage  is  applied  intermittently,  and 
from  which  the  sewage  is  removed  by  a  system  of  underdrains. 
It  differs  from  broad  irrigation  in  the  character  of  the  material, 
the  care  and  preparation  of  the  bed,  and  the  thoroughness  of  the 
under  drainage.  A  distinctive  feature  of  the  intermittent  sand 


Staffed 
Vitrified 
Half 


Tvne  i,"  Steel  Mesh  Reinforcement-' 
Mortar  Joint^        ^--Vif.  Clay  Floor  Block 


Type  B. 


TypeC 


Type  D. 


Type  C-E. 


FIG.  174.— Types  of  False  Bottoms  for  Trickling  Filters. 

Eng.  News,  Vol.  74,  p.  5. 

filter  is  the  quality  of  the  effluent  delivered  by  it.  In  a  properly 
designed  and  operated  plant  the  effluent  is  clear,  colorless,  odor- 
less, and  sparkling.  It  is  completely  nitrified,  is  stable  and  con- 
tains a  high  percentage  of  dissolved  oxygen.  It  contains  no 
settleable  solids  except  at  widely  separated  periods  when  a  small 
quantity  may  appear  in  the  effluent.  The  percentage  removal 
of  bacteria  may  be  from  98  to  99  per  cent.  Some  analyses  of 
sand  filter  effluents  are  given  in  Table  89.  The  dissolved  solids, 
the  remaining  bacteria,  and  the  antecedents  of  the  effluent  are 
the  only  differences  between  it  and  potable  water.  An  effluent 
from  an  intermittent  sand  filter  is  the  most  highly  purified 


INTERMITTENT  SAND  FILTER 


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454  FILTRATION  AND  IRRIGATION 

effluent  delivered  by  any  form  of  sewage  treatment.  The 
effluent  can  be  disposed  of  without  dilution,  on  account  of  its 
high  stability.  The  treatment  of  sewage  to  so  high  a  degree  is 
seldom  required,  so  that  the  use  of  intermittent  niters  is  not 
common.  Other  drawbacks  to  their  use  are  the  relatively  large 
area  of  land  necessary  and  the  difficulty  of  obtaining  good  filter 
sand  in  all  localities. 

The  action  in  an  intermittent  sand  filter  is  more  complete 
than  in  other  forms  of  filters  because  a  greater  surface  is  exposed 
to  the  passage  of  sewage  by  the  fine  sand  particles,  and  the 
sewage  is  in  contact  with  the  filtering  material  a  longer  time 
due  to  the  lower  rate  of  filtration  and  the  slow  velocity  of  flow 
through  the  filter.  It  is  essential  that  the  sewage  be  applied 
to  the  bed  intermittently  in  order  that  air  shall  be  entrained  in 
the  filter.  The  period  between  doses  should  not  be  so  long  that 
the  filter  becomes  dry. 

In  the  operation  of  an  intermittent  sand  filter  one  dose  per 
day  is  considered  an  ordinary  rate  of  application,  although 
some  plants  operate  with  as  many  as  four  doses  per  day  per  filter, 
and  others  on  one  dose  at  long  and  irregular  intervals.  It  is 
not  always  necessary  to  rest  the  filter  for  any  length  of  time  unless 
signs  of  overloading  and  clogging  are  shown.  The  intermittent 
dosing  action  may  be  obtained  by  the  action  of  an  automatic 
siphon  as  is  described  in  Chapter  XXI.  The  sewage  is  distributed 
on  the  beds  through  a  number  of  openings  in  the  sides  of  distribut- 
ing troughs  resting  on  the  surface  of  the  filter.  The  sewage  is 
withdrawn  from  the  bottom  of  the  filter  through  a  system  of 
underdrains,  into  which  it  enters  after  its  passage  through  the 
bed.  There  are  no  control  devices  on  the  outlet,  as  the  rate  of 
filtration  is  controlled  by  the  action  of  the  dosing  apparatus 
and  the  rate  at  which  sewage  is  delivered  to  it.  The  action 
of  the  dosing  apparatus  should  respond  quickly  to  variations 
in  sewage  flow.  As  the  doses  are  applied  to  a  sand  filter,  a  mat 
of  organic  matter  or  bacterial  zooglea  is  formed  on  the  surface 
of  the  bed.  The  mat  is  held  together  by  hair,  paper,  and  the 
tenacity  of  the  materials.  It  may  attain  a  thickness  of  J  to  \ 
an  inch  before  it  is  necessary  to  remove  it.  So  long  as  the  filter 
is  draining  with  sufficient  rapidity  this  mat  need  not  be  removed, 
but  if  the  bed  shows  signs  of  clogging,  the  only  cleaning  that 
may  be  necessary  will  be  the  rolling  up  of  this  dried  mat.  It 


INTERMITTENT  SAND  FILTER  455 

is  believed  that  the  greater  portion  of  the  action  in  the  filter 
occurs  in  the  upper  5  to  8  inches  of  the  bed,  but  occasionally  the 
beds  become  so  clogged  that  it  is  necessary  to  remove  f  of  an 
inch  to  2  inches  of  sand  in  addition  to  the  surface  mat,  or  to 
loosen  up  the  surface  by  shallow  plowing  or  harrowing.  The 
necessity  for  such  treatment  may  indicate  that  the  filter  is  being 
overloaded  as  a  result  of  which  the  rate  of  filtration  should  be 
decreased  or  the  preliminary  treatment  should  be  improved. 
The  plowing  of  clogging  material  into  the  bed  should  be  avoided 
as  under  these  conditions  the  final  condition  of  the  bed  will  be 
worse  than  its  condition  when  trouble  was  first  observed. 

In  winter  the  surface  of  the  bed  should  be  plowed  up  into 
ridges  and  valleys.  The  freezing  sewage  forms  a  roof  of  ice 
which  rests  on  the  ridges  and  the  subsequent  applications  of 
sewage  find  their  way  into  the  filter  through  the  valleys  under 
the  ice.  In  a  properly  operated  bed  the  filtering  material  will 
last  indefinitely  without  change.  If  a  filter  is  operated  at  too 
high  a  rate,  however,  although  the  quality  of  the  effluent  may  be 
satisfactory,  it  will  be  necessary  at  some  time  to  remove  the  sand 
and  restore  the  filter. 

The  rate  of  filtration  depends  on  the  character  of  the  influent, 
the  desired  quality  of  the  effluent,  and  the  depth  and  character 
of  the  filtering  material.  Filters  can  be  found  operating  at  rates 
of  50,000  gallons  per  acre  per  day  and  others  at  eight  times  this 
rate.  For  sewage  which  has  had  some  preliminary  treatment,  the 
rate  should  not  exceed  100,000  gallons  per  acre  per  day,  whereas 
the  rate  for  raw  sewage  should  be  less  than  this.  For  rough 
estimates  made  without  tests  of  the  sewage  in  question,  the  rate 
should  not  be  taken  at  more  than  1,000  persons  per  acre.  If  the 
preliminary  treatment  of  the  sewage  has  been  thorough  and  the 
material  of  the  sand  filter  is  coarser  than  ordinary  the  rate  of 
filtration  can  be  high.  For  less  careful  preliminary  treatment 
and  fine  filtering  material  the  rates  must  be  reduced.  The 
sewage  must  undergo  sufficient  preliminary  treatment  to  remove 
large  particles  of  solid  matter  which  would  otherwise  clog  the 
dosing  apparatus  and  the  filter.  This  treatment  should  include 
grit  removal,  screening,  and  some  form  of  tank  treatment. 
Some  plants  have  operated  successfully  with  a  stale  sewage  and 
no  preliminary  treatment,  as  at  Brockton,  Mass.  Septic  tank 
effluent  can  be  treated  successfully  on  an  intermittent  sand 


456  FILTRATION  AND  IRRIGATION 

filter,  but  not  so  satisfactorily  as  the  effluent  from  a  tank 
delivering  a  fresh  sewage. 

The  material  of  the  filter  should  consist  of  clean,  sharp, 
quartz  or  silica  sand  with  an  effective  size  x  of  0.2  to  0.4  mm., 
preferably  about  0.25  to  0.35  mm.,  and  a  uniformity  coefficient 2 
of  2  to  4.  Within  the  limits  mentioned  no  careful  attention 
need  be  given  to  the  size  of  the  material.  Natural  sand  found 
in  place  has  been  underdrained  and  used  successfully  for  sewage 
treatment.  The  size  of  the  sand  is  fixed  by  the  rate  of  filtra- 
tion rather  than  the  bacteriological  action  of  the  filter.  A 
coarse  sand  will  permit  the  sewage  to  pass  through  the  bed  too 
rapidly,  and  a  fine  sand  will  hold  it  too  long  or  will  become 
clogged.  The  same  size  of  material  should  be  used  throughout 
the  bed,  except  that  a  layer  of  gravel  from  6  to  12  inches  thick, 
graded  from  very  small  sizes  to  stones  just  passing  a  2-inch 
ring  should  be  placed  at  the  bottom  to  facilitate  the  drainage  of 
the  bed. 

The  thickness  of  the  sand  layer  should  not  be  less  than  30 
inches  to  insure  complete  treatment  of  the  sewage.  In  shallower 
beds  the  sewage  might  trickle  through  without  adequate  treat- 
ment. Beds  are  ordinarily  made  from  30  to  36  inches  deep, 
but  when  deeper  layers  of  sand  are  found  in  place  there  is  no  set 
limit  to  the  depth  which  may  be  used.  The  shape  and  overall 
dimensions  of  the  bed  should  conform  to  the  topography  of  the 
site  and  the  rate  of  filtration  adopted.  A  plan  and  cross-section 
of  an  intermittent  sand  filter  showing  the  distribution  and 
underdrainage  systems  are  given  in  Fig.  166  and  175. 

The  distribution  system  consists  of  a  system  of  troughs  on 
the  surface  of  the  filter,  laid  out  in  a  branching  form,  as  shown 
in  the  figure.  The  openings  in  the  troughs  should  be  so 
located  that  the  maximum  distance  from  any  point  on  the  bed 
to  the  nearest  opening  should  not  exceed  20  to  30  feet.  If  the 
filters  are  small  enough,  troughs  need  not  be  used,  the  sewage 
being  distributed  from  one  corner,  or  from  mid-points  on  the 
sides.  Where  troughs  are  used  they  should  be  supported  from 

1  The  effective  size  of  sand  is  the  diameter  in  millimeters  of  the  largest 
grain  in  that   10  per  cent,  by  weight,  of  the  material  which  contains  the 
smallest  grains. 

2  The  uniformity  coefficient  is  the  ratio  of  the  diameter  of  the  largest 
particle  of  the  smallest  60  per  cent,  by  weight,  to  the  effective  size. 


INTERMITTENT  SAND  FILTER 


457 


the  bottom  of  the  filter  in  order  to  prevent  uneven  settling  due 
to  the  washing  of  the  sand.  The  openings  in  the  troughs  are  made 
adjustable  by  swinging  gates  as  shown  in  Fig.  176,  or  by  other 
means  so  that  after  the  filter  is  in  operation  the  intensity  of  the 
dose  on  any  portion  of  the  filter  can  be  changed.  The  troughs 
may  be  placed  with  their  bottoms  level  with  the  surface  of  the 
sand  and  with  sides  of  sufficient  height  to  give  the  required  gra- 
dient to  the  water  surface,  or  they  may  be  built  up  above  the  sur- 


Section  on  Line  A-B. 

FIG.  175. — Plan  and  Section  of  an  Intermittent  Sand  Filter  Showing  Central 
Location  of  Control  House. 


face  of  the  filter  and  given  the  required  slope  so  that  the  surface 
of  the  flowing  water  is  parallel  to  the  bottom  of  the  trough. 
In  either  case  a  splash  plate  should  be  placed  at  each  opening, 
so  that  not  less  than  2  feet  of  the  surface  of  the  sand  is  protected 
in  all  directions  from  the  opening.  A  stone  or  concrete  slab 
2  to  4  inches  thick  makes  a  satisfactory  splash  plate.  Either 
wood  or  concrete  may  be  used  for  the  construction  of  the 
troughs.  The  former  is  less  durable,  but  also  less  expensive 


458 


FILTRATION  AND   IRRIGATION 


in  first  cost.  The  capacity  of  the  troughs  may  be  computed 
by  Kutter's  formula  with  the  quantity  to  be  carried  equal  to  the 
maximum  rate  of  discharge  of  the  feeding  siphon,  with  a  reduc- 
tion in  size  below  each  branch  or  outlet  proportional  to  the 
amount  which  will  be  discharged  above  this  point. 

The  operation  of  automatic  devices  for  dosing  the  bed  is 
explained  in  Chapter  XXI.  The  dosing  tank  should  have  a 
capacity  sufficient  to  cover  the  bed  to  a  depth  of  about  1  to  3 
inches  at  one  dose,  and  the  siphon  should  discharge  at  a  rate  of 
about  one  second-foot  for  each  5,000  square  feet  of  filter  area. 
A  dose  should  disappear  within  20  minutes  to  half  an  hour  after 
it  is  applied  to  the  filter.  With  the  rate  stated  and  four  appli- 


FIG. 176. — Distributing  Trough  with  Adjustable  Openings. 

cations  per  day  to  a  depth  of  1  inch  at  each  dose,  the  rate  per 
acre  per  day  will  be  109,000  gallons. 

The  filtration  of  sewage  through  sand  in  a  manner  similar 
to  the  rapid  sand  filtration  of  water  is  being  attempted  at  the 
Great  Lakes  Naval  Training  Station.  No  results  of  this  treat- 
ment have  been  published  and  the  practical  success  of  the  method 
has  not  been  assured. 

259.  Cost  of  Filtration. — Only  comparative  figures  can  be 
given  in  stating  the  costs  of  filtration,  as  most  data  available 
are  based  on  pre-war  conditions,  and  are  therefore  unreliable 
for  present  conditions.  The  variations  from  the  figures  given 
may  be  very  large  but  in  general  the  relative  costs  have  not 
changed.  The  figures  given  in  Table  90  are  suggestive  of  the  rela- 
tive costs  of  the  different  forms,  of  filtration. 


THE  PROCESS 


459 


TABLE  90 

RELATIVE   COSTS   OF   DIFFERENT   METHODS   OF   SEWAGE   TREATMENT 
Costs  in  Dollars  per  Million  Gallons  per  Day 


Operation 

Form  of  Treatment 

First  Cost  * 

and 

Total 

Maintenance 

Coarse  screens 

0  20 

Fine  screens  

3.00 

Plain  sedimentation  

7.00 

3.00 

10.00 

Chemical  precipitation 

22  00  f 

Septic  tank  

7.00 

1.00 

8.00 

Imhoff  tank  

10.00. 

1.00 

11.00 

Contact  bed 

8  00 

2  00 

10  00 

Trickling  filter  

4.00 

2.00 

6  00 

Intermittent  sand  filter  

15.00 

10.00 

25  .  00 

Activated  sludge 

6  50 

8  50 

15  OOt 

*  Interest  at  6  per  cent, 
the  sale  of  sludge. 


f  Worcester  figures.       t  This  method  may  show  a  profit  from 


IRRIGATION 

260.  The  Process. — Broad  irrigation  is  the  discharge  of 
sewage  upon  the  surface  of  the  ground,  from  which  a  part  of 
the  sewage  evaporates  and  through  which  the  remainder  perco- 
lates, ultimately  to  escape  in  surface  drainage  channels.  Sewage 
farming  is  broad  irrigation  practiced  with  the  object  of  raising 
crops.  Broad  irrigation  can  be  accomplished  successfully  with- 
out the  growing  of  crops,  but  it  is  seldom  attempted  as  some 
return  and  sometimes  even  a  profit  can  be  obtained  from  the 
crops  raised.  Broad  irrigation  and  sewage  farming  differ  from 
intermittent  sand  filtration  in  the  intensity  of  the  application 
of  the  sewage,  the  method  of  preparing  the  area  on  which  the 
sewage  is  to  be  treated,  and  the  care  in  operation.  In  broad 
irrigation  and  intermittent  sand  filtration  the  paramount  con- 
sideration is  successful  disposal  of  the  sewage.  In  sewage  farming 
the  paramount  consideration  is  the  growing  of  crops.  The 
growing  of  crops  may  be  combined  with  irrigation  and  filtration, 
however,  but  the  crop  should  be  sacrificed  to  the  successful 
disposal  of  the  sewage. 


460  FILTRATION  AND   IRRIGATION 

The  change  which  occurs  in  the  characteristics  of  the  sewage 
due  to  its  filtration  through  the  ground  is  the  same  as  occurs  in 
aerobic  filtration.  The  effect  on  the  crops  is  mainly  that  of  an 
irrigant,  as  the  manurial  value  of  the  sewage  is  small. 

261.  Status. — The  disposal  of  sewage  by  broad  irrigation  was 
practiced  in  England  previous  to  the  development  of  any  of  the 
more  intensive  biologic  methods  of  treatment.  It  was  con- 
sidered the  only  safe  and  sanitary  method  for  the  disposal  of 
sewage,  and  as  a  result,  areas  irrigated  by  sewage  were  common 
throughout  England.  Crops  were  grown  on  these  areas  as  a 
minor  consideration,  and  sewage  farming  gained  some  of  its 
popularity  from  the  apparent  success  of  these  disposal  areas. 
The  success  of  sewage  farms  is  due  more  to  generous  irrigation 
in  dry  years  than  to  fertilization  by  sewage. 

The  sewage  farms  of  Paris  and  Berlin  are  frequently  cited 
as  examples  of  the  successful  and  remunerative  disposal  of 
sewage  by  farming  in  connection  with  broad  irrigation.  Kinni- 
cutt,  Winslow,  and  Pratt l  state: 

The  Berlin  Sewage  farms  offer  examples  of  broad 
irrigation  under  better  conditions  ...  of  21,008  acres 
receiving  sewage,  16,657  acres  were  farmed  by  the  city, 
3,956  acres  were  leased  to  farmers,  and  only  395  acres 
were  unproductive.  The  contributing  population  at  this 
time  was  2,064,000  and  the  average  amount  of  sewage 
treated  was  77,000,000  gallons,  giving  a  daily  rate  of 
treatment  of  about  3,700  gallons  per  acre  of  prepared 
land.  The  soil  is  sandy  and  of  excellent  quality.  A  quarter 
of  the  area  operated  by  the  authorities  is  devoted  to 
pasturage,  and  about  a  third  to  the  cultivation  of  cereals, 
of  which  winter  rye  and  oats  are  the  most  important. 
Potatoes  and  beets  are  grown  in  considerable  amounts 
and  a  wide  variety  of  other  crops  in  smaller  proportions. 
.  .  .  Even  fish  ponds  are  made  to  yield  a  part  of  the 
revenue,  and  the  drains  on  some  of  the  farms  have  been 
successfully  stocked  with  breed  trout. 

The  cost  of  the  Berlin  farms  to  March  31,  1910, 
was  $17,470,000,  somewhat  more  than  half  being  the 
purchase  price  of  the  land.  The  expenses  for  this  year 
amounted  to  $1,300,385  for  maintenance,  and  $741,818 
for  interest  charges.  The  receipts  were  $1,240,773  and 
there  was  an  estimated  increase  of  $122,593  in  value 
of  live  stock  and  other  property. 

1  Sewage  Disposal,  1919,  p.  223. 


PREPARATION  AND  OPERATION  461 

The  conditions  at  Berlin  are  quoted  at  length  to  indicate  the 
success  which  can  accompany  broad  irrigation,  and  as  an 
example  of  what  is  being  done  abroad,  where  the  rainfall  is  light 
and  the  soil  is  suitable. 

In  the  United  States  success  in  sewage  farming  has  not  been 
marked.  This  may  be  due  partially  to  the  relative  weakness  of 
American  sewages,  to  the  cost  of  labor,  to  lack  of  satisfactory 
irrigation  areas,  and  to  inattention  to  details.  An  attempt  was 
made  to  grow  crops  on  the  sand  filters  at  Brockton,  Mass.,  but 
it  was  finally  abandoned  as  the  interests  of  the  crops  and  the 
successful  treatment  of  the  sewage  could  not  both  be  satisfied. 
At  Pullman,  Illinois,1  in  1880,  there  was  commenced  probably 
the  most  extensive  attempt  at  sewage  farming  in  eastern 
United  States.  The  farm  was  a  failure  from  the  start,  because 
of  the  clay  soil,  and  it  was  subsequently  abandoned.  Sewage 
farming,  mainly  as  a  subsidiary  consideration  to  the  filtration 
of  sewage,  is  practiced  in  a  few  cities  in  the  eastern  portion  of 
the  United  States  to-day.  Among  the  cities  mentioned  by 
Met  calf  and  Eddy  2  are  Danbury,  Conn.,  and  Fostoria,  Ohio. 
In  the  western  portion  of  the  United  States  where  water  is 
scarce  and  the  ground  is  porous,  sewage  has  been  used  as  an  irri- 
gant  with  some  success.  Such  use  of  sewage  cannot  be  considered 
as  a  method  of  treatment  since  the  prime  consideration  is  the 
growing  of  crops.  In  this  process  all  sewage  not  used  as  an 
irrigant  is  discharged  without  treatment  into  water  courses. 
According  to  Met  calf  and  Eddy  there  were  35  cities  in  Cali- 
fornia in  1914  that  were  operating  sewage  farms.  Among 
these  are  Pasadena,  Fresno,  and  Pomona.  Other  farms,  notably 
the  pioneer  farm  at  Cheyenne,  Wyo.,  have  been  abandoned 
because  of  the  local  nuisance  created  and  the  lack  of  financial 
success. 

262.  Preparation  and  Operation. — A  porous  sandy  soil  on 
a  good  slope  and  with  good  underdrainage  is  most  suitable  for 
broad  irrigation.  Impervious  clay  or  gumbo  soils  are  unsuitable 
and  should  not  be  used.  They  become  clogged  at  the  surface, 
forming  pools  of  putrefying  sewage,  or  in  hot  weather  form  cracks 
which  may  permit  untreated  sewage  to  escape  into  the  underdrains. 

The  sewage  may  be  distributed  to  the  irrigated  area  in  any 

1  See  Eng.  News,  Vol.  9,  1883,  p.  203,  and  Vol.  29,  1893,  p.  27. 

2  American  Sewerage  Practice,  Vol.  III. 


462  FILTRATION  AND  IRRIGATION 

one  of  five  ways  which  are  known  as:  flooding,  surface  irriga- 
tion, ridge  and  furrow  irrigation,  filtration,  and  sub-surface 
irrigation.  In  flooding,  sewage  is  applied  to  a  level  area 
surrounded  by  low  dikes.  The  depth  of  the  dose  may  be  from 
1  inch  to  2  feet.  In  surface  irrigation  the  sewage  is  allowed  to 
overflow  from  a  ditch  over  the  surface  of  the  ground  into  which 
it  sinks  or  over  which  it  flows  into  another  ditch  placed  lower 
down.  This  ditch  conducts  it  to  a  point  of  disposal  or  to  another 
area  requiring  irrigation.  Ridge  and  furrow  irrigation  consists 
in  plowing  a  field  into  ridges  and  furrows  and  filling  the  furrows 
with  sewage  while  crops  are  grown  on  or  between  the  ridges. 
In  filtration  the  sewage  is  distributed  in  any  desired  fashion  on 
the  surface  and  is  collected  by  a  system  of  underdrains  after  it 
has  filtered  through  the  soil.  In  subsurface  irrigation  the  sewage 
is  applied  to  the  land  through  a  system  of  open  joint  pipes  laid 
immediately  below  the  surface,  similarly  to  a  system  of  under- 
drains. Combinations  of  and  modifications  to  these  methods 
are  sometimes  made.  Underdrains  may  be  used  in  connection 
with  any  of  these  forms  of  distribution. 

The  preparation  of  the  ground  consists  in:  the  construction 
of  ditches  or  dikes  to  permit  of  any  of  the  above  described 
methods  of  application,  grading  of  the  surface  to  prevent 
pooling,  the  laying  of  underdrains,  and  the  grubbing  and  clearing 
of  the  land.  The  main  carriers  may  be  excavated  in  open  earth 
or  earth  lined  with  an  impervious  material.  The  distribution 
of  the  sewage  from  the  main  carriers  to  groups  of  laterals  may 
be  controlled  by  hand-operated  stop  planks.  If  the  soil  has  a 
tendency  to  become  waterlogged  it  may  be  relieved  by  install- 
ing underdrains  at  depths  of  3  to  6  feet,  and  40  to  100  feet 
apart.  The  tile  underdrains  may  discharge  into  open  ditches 
excavated  for  the  purpose  which  serve  also  to  drain  the  land. 
Drains  should  be  used  where  the  ground  water  is  within  4  feet 
of  the  surface,  and  the  open  ditches  should  be  cut  below  the 
drains  to  keep  the  ground  water  out  of  them.  Four  or  6-inch 
open-joint  farm  tile  may  be  used  for  underdrains.  The  porosity 
of  the  soil  will  be  increased  by  cultivation.  Where  particular 
care  is  taken  in  the  cultivation  of  the  soil  so  that  sewage  can 
be  applied  at  a  high  rate,  broad  irrigation  merges  into  the  more 
intensive  intermittent  filtration  through  sand. 

Before  being  turned  on  to  the  land,  sewage  should  be  screened 


SANITARY  ASPECTS  463 

and  heavy-settling  particles  should  be  removed.  The  rate  of 
application  may  be  increased  as  the  intensity  of  the  preliminary 
treatment  is  increased.  The  rate  at  which  sewage  may  be 
applied  is  dependent  also  on  the  character  of  the  soil,  and  may 
vary  between  4,000  and  30,000  gallons  per  acre  per  day,  although 
higher  rates  have  been  used  with  the  effluent  from  treatment 
plants  and  on  favorable  soil.  The  sewage  should  be  applied 
intermittently  in  doses,  the  time  between  doses  varying  between 
one  day  and  two  or  three  weeks  or  more,  dependent  on  the 
weather  and  the  condition  of  the  soil.  The  methods  of  dosing 
vary  as  widely  as  the  rates.  The  dose  may  be  applied  con- 
tinuously for  one  or  two  weeks  with  correspondingly  long  rests, 
or  it  may  be  applied  with  frequent  intermittency  alternated 
with  short  rests,  interspersed  with  long  rest  periods  at  longer 
intervals  of  time.  When  applying  the  sewage  to  the  land  the 
rate  of  application  of  the  dose  is  about  10,000  to  150,000  gallons 
per  acre  per  day.  The  area  under  irrigation  at  any  one  time 
may  be  as  much  as  10  to  15  acres.  The  rate  of  the  application 
of  the  sewage  is  also  dependent  on  the  weather  and  may  vary 
widely  between  seasons.  It  is  obvious  that  a  rain-soaked 
pasture  cannot  receive  a  large  dose  of  sewage  without  danger 
of  undue  flooding.  One  of  the  principal  difficulties  with  the 
treatment  or  disposal  of  sewage  by  broad  irrigation  is  that  the 
greatest  load  of  sewage  must  be  cared  for  in  wet  seasons  when  the 
ground  is  least  able  to  absorb  the  additional  moisture. 

263.  Sanitary  Aspects. — A  well-operated  sewage  farm  should 
cause  no  offense  to  the  eye  or  nose,  and  is  not  a  danger  to  the 
public  health.     In  Berlin,  a  portion  of  the  sewage  farms  are 
laid  out  as  city  parks.     The  liquid  in  the  drainage  ditches  or 
underdrains  may  be  clear,  odorless,  and  colorless,  high  in  nitrates 
and    non-putrescible.     Where    the    farm    has    been    improperly 
managed  or  overdosed  the  condition  may  be  serious  from  both 
esthetic    and    health    considerations.     Sewage    may    be    spread 
out  to  pollute  the  atmosphere  and  to  supply  breeding  places  for 
flying  insects  which  will  spread  the  filth  for  long  distances  sur- 
rounding the  farm.     The  character  of  the  crop  is  also  a  sani- 
tary consideration. 

264.  The  Crop. — From  a  sanitary  viewpoint  no  crops  which 
come  in  contact  with  the  sewage  should  be   cultivated  on  a 
sewage  farm.     Such  products  as  lettuce,  strawberries,  asparagus, 


464  FILTRATION  AND  IRRIGATION 

potatoes,  radishes,  etc.,  should  not  be  grown.  Grains,  fruits, 
and  nuts  are  grown  successfully  and  as  they  do  not  come  in  con- 
tact with  the  sewage  there  is  no  sanitary  objection  to  their 
cultivation  in  this  manner.  Italian  rye  grass  and  other  forms 
of  hay  are  grown  with  the  best  success  as  they  will  stand  a 
large  amount  of  water  without  injury.  The  raising  of  stock 
is  also  advisable  for  sewage  farms  where  hay  and  grain  are  culti- 
vated. The  stock  should  be  fed  with  the  fodder  raised  on  the 
irrigated  lands  and  should  not  be  allowed  to  graze  on  the  crops 
during  the  time  that  they  are  being  irrigated.  This  is  due  as 
much  to  the  danger  of  injury  to  the  distributing  ditches  and 
the  formation  of  bogs  by  the  trampling  of  the  cattle,  as  to  the 
danger  to  the  health  of  the  cattle. 


CHAPTER  XVIII 
ACTIVATED   SLUDGE 

265.  The   Process. — In   the   treatment    of   sewage   by   the 
activated  sludge  process  the  sewage  enters  an  aeration  tank  after 
it  has  been  screened  and  grit  has  been  removed.     As  it  enters 
the  aeration  tank  it  is  mixed  with  about  30  per  cent  of  its 
volume  of  activated   sludge.     The   sewage   passes  through  the 
aeration  tank  in  about  two  to  four  hours  during  which  time  air 
is   blown   through   it   in   finely   divided   bubbles.     The   effluent 
from  the  aeration  tank  passes  to  a  sedimentation  tank  where  it 
remains  for  one-half  an  hour  to  an  hour  to  allow  the  sedimen- 
tation of  the  activated  sludge.     The  supernatant  liquid  from  the 
sedimentation  tank  is  passed  to  the  point  of  final  disposal.     A 
portion  of  the  sludge  removed  from  the  tank  is  returned  to  the 
influent  of  the  aeration  tank.     The  remainder  may  be  sent  to 
any  or  all  of  the  following:   the  sludge  drying  process,  the  reaera- 
tion  tanks,  or  to  some  point  for  final  disposal.     Sections  of  the 
activated  sludge  plant  at  Houston,  Texas,  are  shown  in  Fig.  177. 

The  biological  changes  in  the  process  occur  in  the  aeration 
tank.  These  changes  are  dependent  on  the  aerobic  organisms 
which  are  intensively  cultivated  in  the  activated  sludge.  When 
placed  in  intimate  contact  with  fresh  sewage,  brought  about  by 
the  agitation  caused  by  the  rising  air,  and  in  the  presence  of  an 
abundance  of  oxygen,  the  organic  matter  is  partially  oxidized. 
The  putrefactive  stage  of  the  organic  cycle  is  avoided.  Col- 
loids and  bacteria  are  partially  removed  probably  by  the  agita- 
tion effected  in  the  presence  of  activated  sludge  but  the  exact 
action  which  takes  place  is  not  well  understood. 

266.  Composition. — Activated  sludge  is  the  material  obtained 
by  agitating  ordinary  sewage  with  air  until  the  sludge  has  assumed 
a    flocculent    appearance,    will    settle    quickly,     and    contain 
aerobic  and  facultative  bacteria  in  such  numbers  that  similar 
characteristics    can    be    readily    imparted    to    ordinary    sewage 

465 


466 


ACTIVATED  SLUDGE 


sludge  when  agitated  with  air  in  the  presence  of  activated  sludge. 
Copeland  described  activated  sludge  as  follows:1 

The  sludge  embodied  in  sewage  and  consisting  of 
suspended  organic  solids',  including  those  of  a  colloidal 
nature,  when  agitated  with  air  for  a  sufficient  period 
assumes  a  flocculent  appearance  very  similar  to  small 
pieces  of  sponge  Aerobic  and  facultative  bacteria  gather 
in  these  flocculi  in  immense  numbers — from  12  to  14 


/Sludge  Return  Channel^ 


8"Sluice 


Part  Plan  of  Outside  Unit  Aeration  and  Settling  Tank. 

' 


spi 

:  niffirl 

MlM 

dawr/flS// 

t23a« 

CHAMBER    f 
te^/-^'-1 

SLUDGE 
TANK 

<r-d<~> 

H£^ 

^vo 

U^.vAU.^vJ 

^_^^  v^v  ^ 

«  58-  6  "for  North  <5ide  Plank  >i| 
«.  S9-0"  »   South   »        «     -*) 

Half  Section  through  Aeration  Tanks 

Half  Sec-tion  through  Settling  Tanks. 


FIG.  177. — Activated  Sludge  Plant  at  Houston,  Texas. 

Eng.  News,  Vol.  77,  p.  236. 

million  per  c.c. — some  having  been  strained  from  the 
sewage  and  others  developed  by  natural  growth.  Among 
the  latter  are  species  that  have  the  power  to  decompose 
organic  matter,  especially  of  an  albuminoid  or  nitrog- 
enous nature,  setting  the  nitrogen  free;  and  others 
absorbing  the  nitrogen  convert  it  into  nitrites  and 
nitrates.  These  biological  processes  require  time,  air, 
and  favorable  environment  such  as  suitable  temperature, 

1  Reference  11,  at  end  of  this  chapter. 


COMPOSITION 


467 


food  supply  and  sufficient  agitation  to  distribute  them 
throughout  all  parts  of  the  sewage. 

Ardern  states  that  the  sludge  differs  entirely  from  the  usual 
tank  sludge.  It  is  inoffensive  and  flocculent  in  character.  The 
percentage  of  moisture  is  from  95  to  99  per  cent.  American 
experience  has  generally  been  that  the  sludge  does  not  readily 
separate  from  its  moisture  by  treatment  on  fine-grain  filters, 
but  the  results  in  England  and  at  Milwaukee,  Wisconsin,  are  in 
conflict  with  this  general  experience.  Upon  standing  24  hours 
or  more  partially  dried  activated  sludge  may  start  to  decompose 
accompanied  by  the  production  of  offensive  odors. 

Duckworth  states : 

The  activated  sludge  at  Salford  contained  three  times 
as  much  nitrogen,  twice  as  much  phosphoric  acid  and 
one-half  as  much  fa  .y  matter  as  ordinary  sludge. 

TABLE  91 
COMPOSITION  OF  SEWAGE,  IMHOFF  SLUDGE,  AND  ACTIVATED  SLUDGE  AND 

EFFLUENT  AT  MILWAUKEE 
(W.  R.  Copeland,  Eng.  News,  Vol.  76,  p.  665) 


Parts  per  Million 

& 

Nitrogen  as 

Period  of 

Source  of 

03 
g 

Nitrogen    Reported 
as  Ammonia  on  a 

Test 

Sample 

"8 

.5 
e 

1-3 

c  o 

1 

Basis     of     Sludge 
Dried    to    10    Per 

d 

g 

fa 

°+3 

§ 

S 

Cent       Moisture. 

1 

§ 

>J 

II 

& 

Three  samples  of 
Sludge 

02 

£ 

•< 

6 

z 

2 

Aug.,  1915. 

Sewage  

253 

14.6 

7.88 

29 

0.15 

0.13 

Imhoff  effluent.  . 

105 

16.2 

6.10 

27 

0.19 

0.13 

2.87 

3.82 

Activated  sludge 

effluent  

14 

3.8 

3.19 

6 

0.29 

6.00 

5.71 

4.97 

7.04 

Sept.,  1915. 

Sewage  

300 

13.5 

8.81 

29 

0.25 

0.14 

Imhoff  effluent.  . 

116 

15.4 

7.10 

27 

0.12 

0.09 

3.88 

Activated  sludge 

• 

effluent  

8 

5.7 

2.22 

9 

0.24 

5.01 

8.69 

9.00 

These  results  have  been  roughly  checked  by  American  experi- 
menters as  shown  in  Table  9 1.1  In  the  recovery  of  nitrogen 
from  sewage  the  activated  sludge  process  is  the  most  promising 
for  satisfactory  results.  In  all  other  processes  of  sewage  treat- 

1  Reference  15. 


468 


ACTIVATED  SLUDGE 


ment  the  sludge  is  digested  to  some  extent  and  nitrogen  lost  in 
the  gases  or  in  the  soluble  matter  which  passes  off  with  the 
effluent.  In  the  activated  sludge  process  a  negligible  amount 
of  gasification  and  liquefaction  take  place  and  only  a  small 
amount  of  nitrogen  passes  off  with  the  effluent  as  compared  with 
the  loss  from  the  Imhoff  process  as  shown  in  Table  91.  The 
percentage  of  nitrogen  in  dried  activated  sludge  is  shown  in 
Table  92. 

TABLE  92 

NITROGEN  CONTENT  OF  DRY  ACTIVATED  SLUDGE  AND  SLUDGE 
FROM  OTHER  PROCESSES 

(G.  W.  Fuller,  Eng.  News,  Vol.  76,  p.  667) 


Source 

Per  Cent 
Nitrogen 

Milwaukee  (Copeland)  
Manchester,  England  (Ardern) 

4.40 
4  60 

Salford,  England  (Melling)  

3.75 

Urbana,  Illinois  (Bartow)  
Armour  and  Co.  (Noble)  

3.5to  6.4 
4.6 

Approximate  range  of  all  other  processes  . 

l.OtoS.O 

These  figures  are  expressed  in  terms  of  nitrogen  and  not  of  ammonia.     Nitrogen  is  only 
82  per  cent  of  the  ammonia  content, 

Nitrifying  bacteria  and  other  species  which  have  the  power 
of  destroying  organic  matter  have  been  isolated  from  the  sludge. 
An  analysis  of  the  dried  sludge  at  Urbana  l  showed  the  following 
results  after  the  weight  had  been  reduced  95.5  per  cent  by  drying: 
6.3  per  cent  nitrogen,  4.00  per  cent  fat,  1.44  per  cent  phosphorus, 
and  75  per  cent  volatile  matter  or  loss  on  ignition.  Analyses 
of  other  domestic  sewages  have  not  shown  such  high  contents 
of  these  desirable  constituents. 

The  dewatering  of  activated  sludge  is  a  problem  which  offers 
serious  obstacles  to  the  successful  operation  of  the  process.  It 
is  its  greatest  disadvantage.  Five  to  ten  times  the  volume  of 
sludge  may  be  produced  by  the  activated  sludge  process  as  by 
an  Imhoff  tank,  and  the  activated  sludge  contains  a  greater 
percentage  of  water.  According  to  Copeland: 

1  Reference  2. 


ADVANTAGES  AND  DISADVANTAGES  469 

The  best  information  now  available  points  to  a 
combination  of  settling  and  decantation  as  a  preliminary 
dewatering  process.  By  this  means  the  water  will  be 
cut  down  from  about  99  per  cent  to  96  per  cent.  On 
passing  the  concentrated  residue  through  a  pressure 
filter  the  moisture  can  be  cut  down  to  75  per  cent.  The 
press  cake  can  be  dewatered  in  a  heat  drier  to  10  per  cent 
moisture  or  less, 1 

The  quantity  of  sludge  produced  at  Milwaukee  2  is  about  15 
cubic  yards  per  million  gallons  of  sewage,  the  sludge  having 
about  98  per  cent  moisture.  On  the  basis  of  10  per  cent  mois- 
ture it  produces  \  ton  of  dry  sludge  per  million  gallons  of  sewage 
treated.  At  Cleveland,3  20  cubic  yards  per  million  gallons 
at  97.5  per  cent  moisture  are  produced.  Methods  of  drying 
sludge  are  discussed  in  Chapter  XX. 

Chemical  analyses  and  biological  tests  indicate  that  the 
fertilizing  value  of  the  sludge  is  appreciable.  Professor  C.  B. 
Lipman  states,  as  the  result  of  a  series  of  tests  in  which  a  sludge 
and  a  soil  were  incubated  for  one  month,  as  follows:4 

The  amounts  of  nitrates  produced  in  one  month's 
incubation  from  the  soil's  own  nitrogen  and  from  the 
nitrogen  from  the  sludge  mixed  with  the  soil  in  the  ratio 
of  one  part  of  sludge  to  100  of  soil  is,  in  milligrams  of 
nitrate,  as  follows:  Anaheim  soil  without  sludge  6.0, 
with  sludge  10.0;  Davis  soil  without  sludge  4.2,  with 
sludge  14.0;  Oakley  soil  without  sludge  2.2,  with  sludge 
4.0. 

The  effect  of  the  sludge  on  plant  growth  is  shown  in  Table  93. 5 
The  results  represent  the  growth  obtained  after  fifteen  weeks 
from  the  planting  of  30  wheat  seeds  in  each  pot. 

267.  Advantages  and  Disadvantages. — Some  of  the  advantages 
of  the  process  are :  a  clear,  sparkling,  and  non-putrescible  effluent 
is  obtained;  the  degree  of  nitrification  is  controllable  within 
certain  limits;  the  character  of  the  effluent  can  be  varied  to 
accord  with  the  quantity  and  character  of  the  diluting  water 

1  For  mechanical  methods  of  drying  sludge,  see  Reference  22,  p.  1127, 
and  No.  33,  p.  843. 

2  Reference  10. 

3  Reference  13. 

4  University  of  California,  Bulletin  251,  1915. 
6  Reference  25. 


470 


ACTIVATED  SLUDGE 


available;  more  than  90  per  cent  of  the  bacteria  can  be  removed; 
the  cost  of  installation  is  relatively  low;  and  the  sludge  has  some 
commercial  value. 

TABLE  93 

FERTILIZING  VALUE  OF  ACTIVATED  SLUDGE 
(E.  Bartow,  Journal  Am.  Water  Works  Ass'n,  Vol.  3,  p.  327) 


Cultivating  Medium 

Grams  Contained  in  Experimental  Pot 

1 

2 

3 

4 

White  sand 

19,820 
60 
6 
3 
0 

0 
0 

19,820 
60 
6 
3 
0 

0 
8.61 

19,820 
60 
6 
3 
20 

0 
0 

19,820 
60 
6 
3 
0 

20 
0 

Dolomite  
Bone  meal  .  . 

Potassium  sulphate  

Activated  sludge  
Activated  sludge  extracted 
with  Ligroin  

Dried  blood  

Number  of  heads  of  wheat. 
Number  of  seeds  
Weight  of  seeds,  grams  .... 
Bushels  per  acre,  calculated. 
Average   length   of   stalk, 
inches                      

14 
85 
2.38 
6.20 

19.40 
2.25 
0.18 

15 
189 
5.29 
13.6 

23.0 

8.25 
0.68 

22 
491 
13.748 
35.9 

35.4 
26.75 
2.23 

23 
518 
14.504 

38.7 

37.1 

26.21 
2.18 

Weight  of  straw,  grams.  .  . 
Tons  per  acre,  calculated  .  .  . 

Among  the  disadvantages  of  the  process  can  be  included, 
uncertainty  due  to  the  lack  of  information  concerning  the 
results  to  be  expected  under  all  conditions,  high  cost  of  operation 
under  certain  conditions,  the  necessity  for  constant  and  skilled 
attendance,  and  the  difficulty  of  dewatering  the  sludge. 

268.  Historical. — The  most  notable  work  in  the  aeration  Of 
sewage  within  recent  years  was  that  performed  by  Black  and 
Phelps  for  the  Metropolitan  Sewerage  Commission  of  New  York, 
in  1910,1  and  by  Clark  and  Gage  at  the  Lawrence,  Massachusetts, 
Sewage  Experiment  Station  in  1912  and  1913. 2  The  results  of 

1  See  Report  by  Black  &  Phelps  of  Metropolitan  Sewerage  Commission, 
1911,  reprinted  as  Vol.  VII  of  Contributions  from  the  Sanitary  Research 
Laboratory  of  the  Massachusetts  Institute  of  Technology. 

2  See  Reports,  Mass.  State  Board  of  Health. 


AERATION  TANK  471 

these  investigations  showed  that  the  treatment  of  sewage  by 
forced  aeration  might  give  a  satisfactory  effluent,  but  that  the 
time  and  expense  in  connection  thereto  rendered  the  method 
impractical. 

It  remained  for  Messrs.  Ardern  and  Lockett  of  Manchester, 
England,  to  introduce  the  process  of  the  aeration  of  sewage  in 
the  presence  of  activated  sludge,  as  a  result  of  their  connection 
with  Dr.  Fowler,  who  attributes  his  inspiration  to  his  visit  to 
the  Lawrence  Experiment  Station  and  observing  the  work  of 
Clark  and  Gage.  Ardern  and  Lockett  commenced  their  experi- 
ments in  1913.  Their  results  were  published  in  the  Journal 
of  the  Society  of  Chemical  Industry,  May  30,  1914,  Vol.  33,  p.  523. 
Shortly  thereafter  experiments  were  started  at  the  University 
of  Illinois  by  Dr.  Edw.  Bartow  and  Mr.  F.  W.  Mohlmann  of  the 
Illinois  State  Water  Survey.  At  about  the  same  time  an  experi- 
mental plant  was  started  at  Milwaukee,  by  T.  C.  Hatton,  Chief 
Engineer  of  the  Milwaukee  Sewerage  Commission.  The  United 
States  Public  Health  Service  became  actively  interested  in 
December,  1914,  and  on  February  20,  1915,  announced  its 
intention  to  co-operate  with  the  Baltimore  Sewerage  Commission 
in  the  conduct  of  experiments.  In  May,  1915,  patent  number 
1,139,024  was  granted  to  Leslie  C.  Frank,  Sanitary  Engineer 
of  the  U.  S.  Public  Health  Service,  covering  certain  features  of 
the  process.  Mr.  Frank  generously  donated  this  patent  to  the 
public  for  the  use  of  municipalities. 

The  first  full  sized  plant  for  the  treatment  of  sewage  by 
this  method  was  erected  in  Milwaukee  in  December,  1915.  This 
plant  had  a  capacity  of  1,600,000  gallons  per  day.  It  was  used 
for  experimental  purposes  and  is  not  now  in  use.  The  Champaign, 
Illinois,  septic  tank,  among  the  first  of  its  kind  in  the  country, 
was  converted  into  an  activated  sludge  tank  on  April  13,  1916. 
The  changes,  developments,  and  the  results  obtained  from  these 
and  other  plants  have  been  reported  in  the  technical  press  from 
time  to  time. 

269.  Aeration  Tank. — The  sewage  on  leaving  the  screen  and 
grit  chamber  enters  the  aeration  tank,  which  is  usually  operated 
on  the  continuous-flow  principle,  although  in  the  early  days  of 
experimentation  the  fill-and-draw  method  was  practiced.  This 
tank  should  be  rectangular  with  a  depth  of  about  15  feet  and  a 
width  of  channel  not  to  exceed  6  to  8  feet.  Such  proportions 


472  ACTIVATED  SLUDGE 

allow  better  air  and  current  distribution  than  larger  tanks. 
The  bottom  should  be  level  to  insure  an  even  distribution  of 
air.  The  velocity  of  flow  of  sewage  through  the  tank  is  usually 
in  the  neighborhood  of  5  feet  per  minute,  dependent  on  the 
length  of  the  tank  and  the  period  of  retention.  The  period  of 
retention  is  in  turn  dependent  on  the  desired  quality  of  the 
effluent.  The  process  is  flexible  and  the  quality  of  the  effluent 
can  be  changed  by  changing  the  period  of  retention  or  by  changing 
the  rate  of  application  of  the  air,  or  both.  The  period  of  retention 
in  the  aeration  tank  is  usually  about  4  hours. 

The  bottom  of  the  aeration  tank  is  usually  made  of  concrete 
arranged  in  ridges  and  valleys,  or  small  shallow  hoppers,  at  the 
bottom  of  which  the  air-diffusing  devices  are  located,  as  shown 
in  Fig.  177.  The  inlet  and  outlet  devices  are  similar  to  those 
in  a  plain  sedimentation  tank. 

270.  Sedimentation  Tank. — It  is  evident  that  as  no  sedi- 
mentation is  permitted  in  the  aeration  tank,  the  settleable  parti- 
cles will  be  discharged  in  the  effluent  unless  some  provision  is 
made  for  their  detention.  The  effluent  from  the  aeration  tank 
is  therefore  run  through  a  plain  sedimentation  tank,  usually 
with  a  hopper  bottom,  which  has  been  arranged  to  permit  fre- 
quent and  easy  cleaning.  An  air  lift  or  a  centrifugal  sludge 
pump  is  satisfactory  for  this  purpose.  Another  type  of  sedi- 
mentation tank  which  has  been  used  has  a  smooth  bottom  with 
a  slight  slope  towards  the  center.  A  revolving  scraper  collects 
the  sludge  continuously,  scraping  it  towards  the  center  of  the 
tank.  Although  this  arrangement  gives  better  results  than  the 
hopper-bottom  tank,  its  expense  has  usually  prevented  its  instal- 
lation.1 

The  period  of  sedimentation  in  different  plants  varies  from 
30  minutes  to  one  hour,  although  the  longer  periods  usually  give 
the  better  results.  Approximately  65  per  cent  of  the  sludge  will 
settle  in  the  first  10  minutes,  80  per  cent  in  the  first  30  minutes, 
and  about  5  per  cent  more  in  the  next  half  hour. 

The  effluent  from  the  sedimentation  tank  is  ready  for  final 
disposal  or  if  desired,  for  further  treatment  by  some  other 
method.  The  sludge,  or  a  portion  of  it,  is  pumped  back  into  the 
influent  of  the  aeration  tank,  provided  the  sludge  is  in  a  satis- 
factory state  of  nitrification.  Otherwise  it  should  be  pumped 

1  Reference  47. 


THE  REAERATION  TANK  473 

to  the  reaeration  tanks.  The  remainder  of  the  sludge  which  is 
not  to  be  used  in  the  process  is  ready  for  drying  and  final 
disposal. 

271.  Reaeration  Tank. — The   purpose   of  the   reaeration  or 
sludge  aeration  tank  is  to  reactivate  the  sludge  which  has  gone 
through  the  aeration  tank.     During  the  process  of  the  aeration 
of  the  sewage  in  the  aeration  tank  the  activated  sludge  may  lose 
some  of  its   qualities  because   of  the  deficiency  of  oxygen  to 
maintain  aerobic  conditions.     By  blowing  air  through  the  sludge 
in  the  reaeration  tank  these  properties  are  returned  and  the  sludge 
made  available  to  be  pumped  back  into  the  aeration  tank.     The 
reactivation  of  the  sludge  obviates  the  necessity  for  supplying 
sufficient  air  to  the  entire  mass  of  the  sewage  to  maintain  aerobic 
conditions,  and  results  in  an  economy  in  the  use  of  air.     The 
use  of  mechanical  agitators  has  also  been  attempted  both  in  the 
reaeration  and  the  aeration  tanks  with  the  expectation  of  saving 
in  the  use  of  air,  but  with  indifferent  success. 

It  is  difficult  to  say,  without  experimentation,  what  the  size 
of  the  reaeration  tank  should  be,  as  the  necessary  amount  or 
reactivation  is  uncertain.  In  the  experimental  plant  at  Mil- 
waukee, there  were  eight  units  of  aeration  tanks,  one  sedimenta- 
tion tank,  and  two  reaeration  tanks,  all  of  the  same  capacity  and 
general  design.  This  represents  a  ration  of  about  one  reaeration 
tank  to  four  aeration  tanks. 

272.  Air  Distribution. — Air  is  applied  to  the  sewage  at  the 
bottom  of  the  aeration  tank  at  a  pressure  in  the  neighborhood 
of^.S  to  6.0  pounds  per  square  inch,  dependent  on  the  depth  of 
the  sewage,   the  loss  of  head  through  the   distributing   pipes, 
and  the  rate  of  application.     In  different  experimental  plants 
the  pressure  has  varied  from  3  to  30  pounds  per  square  inch. 
Such  pressures  are  on  the  line  which  divides  the  use  of  direct 
blowers  for  low  pressures  from  turbo  and  reciprocating  pressure 
machines  for  pressures  above  10  pounds  per  square  inch.     Posi- 
tive-pressure blowers  or  direct  blowers  operate  on  the  principle 
of  a  centrifugal  pump  and  because  of  the  lighter  specific  gravity 
of  air  they  rotate  at  a  very  high  speed.     The  Nash  Hytor  Turbo 
Blower  consists  of  a  rotor  with  a  large  number  of  long  teeth 
slightly  bent  in  the  direction  of  rotation.     The  rotor,  which  has 
a   circular   circumference,   revolves  in  an  elliptical  casing.     At 
the  commencement  of  operation  the  rotor  and  casing  are  partially 


474 


ACTIVATED  SLUDGE 


filled  with  water.  The  revolution  of  the  rotor  throws  the  water 
to  the  outside  of  the  elliptical  casing  thus  forming  a  partial 
vacuum  between  any  two  teeth  as  the  water  is  thrown  from  near 
the  center  of  the  short  diameter  of  the  casing  to  the  extremity 
of  the  long  diameter  of  the  casing.  Air  is  allowed  to  enter 
through  the  inlet  port  to  relieve  the  vacuum.  As  the  teeth  pass 
from  the  long  diameter  to  the  short  diameter  of  the  ellipse,  the 
water  again  approaches  the  center  of  the  rotor  compressing  the 

air  trapped  between  the 
teeth  and  forcing  ic  out 
under  pressure  into  tne  ex- 
haust pipe.  Among  the  ad- 
vantages of  this  compressor 
are  the  washing  of  the  air, 
cooling,  and  ease  in  opera- 
tion. Reciprocating  air  com- 
pressors operate  similarly  to 
direct-acting  steam  pumps 
or  crank  -  and  -  fly  -  wheel 
pumps  but  at  much  higher 
speeds,  and  they  require 
more  floor  space  than  either 
of  the  other  types.  Fig. 
178  shows  the  field  of 


FIG. 


Volume  Of  Free  Air  in  Thousands 
Cu.  Ft  per  Minu+e 

178. — Economic  Range  of  Air  Com- 
pressors. 

From  Eng.  News,  Vol.  74,  p.  906. 


of     various 
compression 


serviceability 
types  of  air 
machinery. 

For  pressures  up  to  about  10  pounds  per  square  inch  the  posi- 
tive blower  seems  most  desirable.  It  has  a  low  first  cost  and  a 
relatively  high  efficiency  of  about  75  to  80  per  cent  of  the  power 
input.  No  oil  or  dirt  is  added  to  the  air  to  clog  the  distributing 
plates,  as  in  the  reciprocating  machine.  A  disadvantage  is  the 
difficulty  of  varying  the  pressure  or  quantity  of  the  output  of 
the  machine.  As  the  required  pressure  and  volume  of  air 
increases  the  turbo  blower  becomes  more  and  more  desirable 
within  the  limits  of  pressure  which  are  ordinarily  used  in  this 
process.  For  small  installations  the  best  form  of  power  is 
probably  the  electric  drive,  but  when  the  capacity  becomes 
such  as  to  make  turbo  blowers  advisable  they  should  be  driven 
by  directly  connected  steam  turbines. 


AIR  DISTRIBUTION  475  ( 

The  quantity  of  air  required  varies  between  0.5  to  6.0  cubic 
feet  per  gallon  of  sewage,  with  from  3  to  6  hours  of  aeration. 
The  quantity  of  air  depends  on  the  degree  of  treatment  required, 
the  strength  of  the  sewage,  the  depth  of  the  tank,  and  the  period 
of  aeration.  The  deeper  the  tank  the  less  the  amount  of  air 
needed  because  of  the  greater  travel  of  the  bubble  in  passing 
through  the  sewage,  but  the  higher  the  pressure  at  which  the 
air  must  be  delivered.  Shallow  tanks  usually  require  a  longer 
period  of  retention.  The  depth  of  the  tank  then  has  very  little 
to  do  with  economy  in  the  use  of  air.  Hatton  states:1 

The  purification  of  sewage  obtained  varies  decidedly 
with  the  volume  of  air  applied.  Small  volumes  applied 
for  5  or  6  hours  do  as  well  as  larger  volumes  applied  for 
3  or  4  hours,  but  the  time  of  aeration  required  to  obtain 
a  like  effluent  does  not  vary  directly  with  the  volume  of 
air  applied  per  unit  of  time.  For  instance  air  applied 
at  a  rate  of  2  cubic  feet  per  minute  purifies  the  sewage 
in  less  time  than  one  cubic  foot  of  air  per  minute,  but  will 
not  accomplish  an  equal  degree  of  purification  in  half  the 
time, 

It  has  been  found  that  although  a  low  temperature  has  a  dele- 
terious effect  on  the  process,  by  the  use  of  an  additional  quantity 
of  air  good  results  can  be  maintained.  The  effect  of  changing 
the  quantity  of  air  and  the  period  of  aeration  are  shown  in  Table 
94  taken  from  Hatton. 

The  velocity  of  the  air  in  the  pipes  should  be  about  1,000 
feet  per  minute.  There  should  be  relatively  few  sharp  turns 
in  the  line,  and  the  distributing  mains  should  be  arranged  with- 
out dead  ends.  It  is  desirable  to  use  as  little  piping  as  possible 
and  at  the  same  time  to  make  the  travel  of  the  sewage  long  in 
order  to  maintain  a  non-settling  velocity  and  intimate  contact 
with  the  air.  The  piping  should  be  accessible  and  well  provided 
with  valves.  It  should  be  non-corrodible,  particularly  on  the 
inside,  as  flakes  of  rust  will  quickly  clog  the  air  diffusers.  It 
should  drain  to  one  point  in  order  that  it  can  be  emptied  when 
flooded,  as  occasionally  happens. 

It  is  desirable  to  diffuse  the  air  in  small  bubbles  as  by  this 
means  the  greatest  efficiency  seems  to  be  obtained  from  the 
amount  of  air  added,  A  diameter  y^  to  J  of  an  inch  is  approxi- 

1  Reference  10. 


476 


ACTIVATED  SLUDGE 


s 
g 

§ 


PPLICATION  OF  AIR  ON  THE  RE 
Y  THE  ACTIVATED  SLUDGE  PR 

(Milwaukee  Results) 


Parts  per  Million 


l 

11 


^ 

"84 
8  -a 


II 


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00 


AIR  DISTRIBUTION 


477 


mately  the  maximum  limit  for  the  size  of  an  effective  bubble. 
Monel  metal  cloth,  porous  wood  blocks,  open  jets,  paddles,  and 
other  forms  of  diffusers  have  been  tried,  but  none  have  given 
the  satisfaction  of  the  filtros  plate.  The  relative  value  of  dif- 
ferent types  of  diffusers  is  shown  in  Table  95  taken  from  Hatton.1 
The  Filtros  plates  are  a  proprietary  article  manufactured  by 
the  General  Filtration  Company  of  Rochester,  N.  Y.  They 
are  made  of  a  quartz  sand  firmly  cemented  together  and  can  be 

TABLE  95 

COMPARATIVE  RESULTS  FROM  THE  AERATION  OP  SEWAGE  IN  THE  PRESENCE 
OF  ACTIVATED  SLUDGE  WITH  THE  USE  OF  DIFFERENT  DISTRIBUTING 
MEDIA 

(T.  C.  Hatton,  Eng.  Record,  Vol.  73,  p.  255) 


Pounds 

Air, 

Per 

Nitrates, 

Stability 

Diffusers 

Months  in  1915 

per 
Square 

Cubic 
Feet  per 

Cent 
Bacteria 

Parts 
per 

Effluent 
in 

Inch 

Gallon 

Removed 

Million 

Hours 

Filtros  plate  

June    1  to  Aug.  15 

4.3 

2.06 

91 

3.4 

78 

Air  jet  

June    1  to  Aug.  15 

3.5 

1.94 

91 

2.2 

52 

Filtros  plate  

Nov.  18  to  Dec.     7 

4.6 

1.71 

90 

0.3 

113 

Monel  metal  

Nov.  18  to  Dec.     7 

3.0 

1.71 

80 

0.2 

63 

obtained  with  practically  any  degree  of  porosity,  size  of  pore 
opening  or  dimension  of  plate,  but  they  are  made  in  a  standard 
size  12  inches  square  by  1^  inches  thick.  The  frictional  loss 
through  the  plate  is  not  very  great  for  the  amount  of  air  ordi- 
narily used.  The  plates  are  classified  in  accordance  with  the 
volume  of  air  which  will  pass  through  them,  when  dry,  per 
minute  when  under  a  pressure  of  2  inches  of  water.  These 
classes  run  from  J  to  12  cubic  feet  of  air  per  minute.  The  type 
usually  specified  passes  about  2  cubic  feet  of  air  per  minute. 
The  loss  of  head  through  these  plates  as  tested  at  Milwaukee 
showed  an  initial  loss  of  J  of  a  pound  and  an  additional  loss 
of  about  \  of  a  pound  for  every  cubic  foot  of  air  per  minute  per 
square  foot  of  surface.  It  is  necessary  to  screen  and  wash  the 
air  before  blowing  it  through  the  filtros  plate  as  ordinary  air 
is  so  filled  with  dirt  as  to  clog  the  pores  of  the  diffuser  quite 
rapidly. 

1  Reference  10. 


478  ACTIVATED  SLUDGE 

The  area  of  filtros  plates  required  in  the  bottom  of  the  tank 
is  usually  expressed  in  terms  of  the  free  surface  of  the  tank  or 
as  a  ratio  thereto.  In  the  Urbana  tests  the  best  ratio  was 
found  to  be  less  than  1  :  3  and  more  than  1:9.  In  Milwaukee  1 
the  ratio  adopted  is  in  the  neighborhood  of  1  :  4  or  1  :  5.  At 
Fort  Worth  the  ratio  will  be  about  1  :  7  and  at  Chicago  it  will 
be  1:8.  The  exact  ratio  should  be  determined  by  experiment 
and  will  depend  on  the  construction  of  the  tank  and  the  char- 
acter of  the  raw  sewage  and  the  desired  effluent.  It  is  essential 
that  the  filtros  plates  be  placed  level  and  at  the  same  elevation 
as  otherwise  the  distribution  of  air  will  be  uneven. 

273.  Obtaining   Activated    Sludge. — After   a    plant    is   once 
started  activated  sludge  is  generated  during  the  process  of  treat- 
ment and  with  careful  management  a  stock  of  activated  sludge 
can  be  kept  on  hand.     When  a  plant  is  new,  or  if  shut  down  for 
such  a  length  of  time  that  the  sludge  loses  its  activation,  it  is 
necessary  to  activate  some  new  sludge.     This  is  done  by  blowing 
air   continuously  through   sewage  either  on  the  fill  and   draw 
method  with   periodic   decantations  of  the  supernatant  liquid, 
or  by  the  continuous-flow  process,  but  more  preferably  by  the 
latter.     Where  activated  sludge  is  to  be  obtained  from  fresh 
sewage  alone  the  time  required  is  in  the  neighborhood  of  10  to 
14  days,  and  purification  begins  at  the  start.     An  estimate  of 
the   quantity  which  will  be  obtained   can  not   be  made  with 
accuracy.     After  the  initial  quantity  of  sludge  has  been  obtained 
activated  sludge  can  be  maintained  during  the  process  of  aeration 
of  the  raw  sewage,  or  by  means  of  the  reaeration  tanks  previously 
described. 

The  volume  of  activated  sludge  present  in  the  aeration  tank 
should  be  about  25  per  cent  of  the  volume  of  the  tank.  The 
volume  of  the  sludge  is  measured  in  a  somewhat  arbitrary  manner 
as  the  amount  by  volume  which  will  settle  in  30  minutes  in  an 
ordinary  test  tube.  It  is  found  that  this  is  almost  90  per  cent 
of  the  solids  settling  in  4  to  6  hours. 

274.  Cost. — The   available  information  on  the   cost  of  the 
activated  sludge  process  is  meager  and  unreliable.     The  factors 
entering  into  the  cost  are :  the  price  of  fuel,  the  size  of  the  plant, 
the  period  of  sedimentation,  the  amount  of  air  per  gallon  of  sewage, 
the  air  pressure,  and  the  percentage  of  sludge  to  be  aerated  in  the 

1  Reference  10. 


COST 


479 


mixture.  In  Milwaukee1  the  cost  of  construction  is  estimated 
at  $44,000  per  million  gallons,  and  $4.75  per  million  gallons  for 
operation.  At  Houston,  Texas,  the  cost  is  estimated  at  $24,000 
per  million  gallons,  exclusive  of  the  sludge-drying  plant,  which 
may  cost  $40,000  per  million  gallons.  At  Milwaukee,  the  cost 
of  pressing  the  sludge  is  $4.82  per  dry  ton  and  of  drying  is  $3.93 
per  dry  ton.  The  sludge  may  be  sold  at  the  normal  rate  of  $2.50 
per  unit  of  nitrogen.  Based  on  the  normal  value  the  evident 
profit  will  be  $3.75  per  ton.  The  net  cost  of  disposing  of  Mil- 
waukee sewage  is  estimated  at  $9.64  per  million  gallons  of  which 
$4.89  is  chargeable  to  overhead  and  $4.75  to  repairs,  operation 
and  renewal.  In  a  comparison  of  the  costs  of  activated  sludge 
and  Imhoff  tanks  with  sprinkling  filters,2  the  information  given 
by  Eddy  has  been  summarized  in  Table  96.  In  comparing  the 

TABLE  96 

COMPARATIVE  COSTS  OF  ACTIVATED  SLUDGE,  AND  OP  IMHOFF  TANKS 
FOLLOWED  BY  SPRINKLING  FILTERS 

(H.  P.  Eddy,  Eng.  Record,  Vol.  74,  p.  557) 


Total  Annual  Cost  at 

Process 

First 
Cost  per 
Million 

Operation 
per 
Million 

4    Per    Cent   with 
Sinking    Fund    at 
2.5  Per  Cent  per 

Gallons, 

Gallons, 

Dollars 

Dollars 

Million 
Gallons, 
Dollars 

Capita, 
Dollars 

Activated  sludge 

57  100 

20  00 

29  85 

1  09 

Imhoff  tank  and  sprinkling  filter. 

78,500 

8.50 

21.84 

0.80 

relative  areas  required  for  different  methods  of  sewage  treatment, 
activated  sludge  should  be  allowed  about  15  million  gallons 
per  acre  per  day  on  the  basis  of  aeration  tanks  15  feet  deep. 
This  figure  represents  approximately  the  gross  area  of  the  plants 
at  Milwaukee  and  at  Cleveland. 


1  Hatton,  reference  33. 


2  Reference  18. 


480  ACTIVATED  SLUDGE 


REFERENCES    AND   BIBLIOGRAPHY   ON   ACTIVATED 

SLUDGE 

The  following  abbreviations  will  be  used:  A.S.  for  Activated  Sludge, 
E.G.  for  Engineering  and  Contracting,  E.N.  for  Engineering  News,  E.R. 
for  Engineering  Record,  E.N.R.  for  Engineering  News-Record,  p.  for  page, 
and  V,  for  volume. 

No. 

1.  Cooperation   Sought   in   Conducting   A.S.   Experiments   at   Baltimore, 

by  Franks  and  Hendrick.  E.R.  V.  71,  1915,  pp.  521,  724,  and  784. 
V.  72,  1915,  pp.  23,  and  640. 

2.  Sewage  Treatment  Experiments  with  Aeration  and  A.S.,  by   Bartow 

and  Mohlman.     E.N.  V.  73,  1915,  p.  647,  and  E.R.  V.  71,  1915,  p.  421. 

3.  A.S.  Experiments  at  Milwaukee,  Wisconsin,  by  Hatton.     E.N.  V.  74, 

1915,  p.  134. 

4.  A.S.  in  America,  An  Editorial  Survey,  by   Baker.     E.N.  V.  74,  1915, 

p.  164. 

5.  Choosing  Air  Compressors  for  A.S.,  by  Nordell,  E.N.  V.  74,  1915,  p.  904. 

6.  A  Year  of  A.S.  at  Milwaukee,  by  Fuller.    E.N.  V.  74,  1915,  p.  1146. 

7.  A.S.  Experiments  at  Urbana.     E.N.  V.  74,  1915,  p.  1097. 

8.  Experiments  on  the  A.S.  Process,  by  Bartow  and  Mohlman.     E.G.  V. 

44,  1915,  p.  433. 

9.  Milwaukee's  A.S.  Plant,  the  Pioneer  Large  Scale  Installation,  by  Hat- 

ton.     E.R.  V.  72,  1915,  p.  481  and  E.G.  V.  44,  1915,  p.  322. 

10.  A.S.  Experiments  at  Milwaukee,  by  Hatton.     Journal  American  Water- 

works Association  and  Proceedings  Illinois  Society  of  Engineers,  1916. 
Also  E.R.  V.  73,  1916,  p.  255.  E.G.  V.  45,  1916,  p.  104,  and  E.N. 
V.  75,  1916,  pp.  262  and  306. 

11.  A.S.  Defined.    E.N.  V.  75,  1916,  p.  503,  and  E.N.R.    V.  80,  1918,  p.  205. 

12.  Status  of  A.S.  Sewage  Treatment,  by  Hammond.     E.N.  V.  75,  1916, 

p.  798. 

13.  Trial  A.S.  Unit  at  Cleveland,  by  Pratt.    E.N.  V.  75,  1916,  p.  671. 

14.  Air  Diffuser  Experience  with  A.S.     E.N.  V.  76,  1916,  p.  106. 

15.  Nitrogen    from    Sewage    Sludge,    Plain   and   Activated,    by    Copeland, 

Journal  American  Chemical  Society,   Sept.   28,    1916.     E.N.   V.   76, 

1916,  p.  665.     E.R.  V.  74,  1916,  p.  444. 

16.  Tests  Show  A.S.     Process  Adapted  to  Treatment  of  Stock  Yards  Wastes. 

E.R.  V.  74,  1916,  p.  137. 

17.  Aeration  Suggestions  for  Disposal  of  Sludge,  by  Hammond.     Journal 

American  Chemical  Society,  Sept.  25,  1916.     E.R.  V.  74,  1916,  p.  448. 

18.  Cost  Comparison  of  Sewage  Treatment.  Imhoff  Tank  and  Sprinkling 

Filters  vs.  A.S.,  by  Eddy.    E.R.  V.  74,  1916,  p.  557. 

19.  Large  A.S.  Plant  at  Milwaukee.     E.N.  V.  76,  1916,  p.  686. 

20.  A.S.  Novelties  at  Hermosa  Beach,  Cal.    E.N.  V.  76,  1916,  p.  890. 

21.  A.S.  Experiments  at  University  of  Illinois,  by  Bartow,  Mohlman,  and 

Schnellbach.     E.N.  V.  76,  1916,  p.  972. 


REFERENCES  AND  BIBLIOGRAPHY  481 

No. 

22.  A.S.  Results  at  Cleveland  Reviewed,  by  Pratt  and  Gascoigne.     E.N. 

V.  76,  1916,  pp.  1061  and  1124. 

23.  Sewage  Treatment  by  Aeration  and  Activation,  by  Hammond.     Pro- 

ceedings American  Society  Municipal  Improvements,  1916. 

24.  A.S.,  by  Bartow  and  Mohlman,  Proceedings  Illinois  Society  of  Engineers, 

1916. 

25.  The  Latest  Method  of  Sewage  Treatment,  by  Bartow.     Journal  Ameri- 

can Waterworks  Association,  V.  3,  March,  1916,  p.  327. 

26.  Winter  Experiences  with  A.S.,  by  Copeland.     Journal  American  Society 

of  Chemical  Engineers,  April  21,  1916.     E.G.  V.  45,  1916,  p.  386. 

27.  A.S.  Process  Firmly  Established,  by  Hatton.     E.R.  V.  75,  1917,  p.  16. 

28.  Operate    Continuous    Flow    A.S.    Plant,    by    Bartow,    Mohlman,    and 

Schnellbach.     E.R.  V.  75,  1917,  p.  380. 

29.  Chicago  Stock  Yards  Sewage  and  A.S.,  by  Lederer.     Journal  American 

Society  of  Chemical  Engineers,  April  21,  1916.     E.G.  V.  45,  1916,  p.  388. 

30.  The  Patent  Situation  Concerning  A.S.     E.G.  V.  45,  1916,  p.  208. 

31.  "  Sewage  Disposal  "  by  Kinnicutt,  Winslow,  and  Pratt,  published  by 

John  Wiley  &  Sons.     2d  Edition,  Chapter  12. 

32.  A.S.  Tests  Made  by  California  Cities.     E.N.R.  V.  79,  1917,  p.  1009. 

33.  Conclusions  on  the  A.S.     Process  at    Milwaukee.     Journal   American 

Public  Health  Association,  1917.     E.N.R.  V.  79,  1917,  p.  840. 

34.  Dewatering  A.S.  at  Urbana,  by  Bartow.     Journal  American  Institute 

of  Chemical  Engineers,  1917.     E.N.R.  V.  79,  1917,  p.  269. 

35.  Milwaukee  Air  Diffusion  Studies  in  A.S.     E.N.R.  V.  78,  1917,  p.  628. 

36.  A.S.  Bibliography  (up  to  May  1,  1917)  by  J.  E.  Porter. 

37.  Air  Diffusion  in  A.S.    E.N.R.  V.  78,  1917,  p.  255. 

38.  A.S.  Plant  at  Houston,  Texas.     E.N.  V.  77,  1917,  p.  236,  E.N.R.  83, 

1919,  p.  1003,  and  V.  84,  1920,  p.  75. 

39.  A.S.  Power  Costs,  by  Requardt.     E.N.  V.  77,  1917,  p.  18. 

40.  A.S.  at  San  Marcos,  Texas,  by  Elrod.     E.N.  V.  77,  1917,  p.  249. 

41.  Filtros  Plates  Made  the  Best  Showing  in  Air  Diffuser  Tests.     E.N.R. 

V.  79,  1917,  269. 

42.  Results   of   Experiments   on   A.S.,   by  Ardern   and    Lockett.     Journal 

Society  for  Chemical  Research,  V.  33,  May  30,  1914,  p.  523. 

43.  Final  Plans  at  Milwaukee.     E.N.R.  V.  84,  1920,  p.  990. 

44.  A.S.    Bibliography,    published   by    General    Filtration    Co.,    Rochester, 

N.  Y.,  1921. 

45.  A.S.  at  Manchester,  Eng.  by  Ardern.     Journal  Society  Chemical  Indus- 

try, 1921.     E.G.  V.  55,  1921,  p.  310. 

46.  The  Des  Plaines  River  A.S.  Plant,  by  Pearse.     E.N.R.  V.  88,   1920, 

p.  1134. 

47.  Sewage    Treatment    by    the    Dorr    System,    by    Eagles.     Proceedings, 

Boston  Society  of  Engineers,  1920.     Public  Works  V.  50,  1920,  P.  53. 


CHAPTER  XIX 

ACID   PRECIPITATION,   LIME    AND    ELECTRICITY,   AND 
DISINFECTION 

275.  The  Miles  Acid  Process. — The  Miles  Acid  Process  for 
the  treatment  of  sewage  was  devised  and  patented  by  G.  W. 
Miles.  It  was  tried  experimentally  at  the  Calf  Pasture  sewage 
pumping  station,  Boston,  Mass.,  1911  to  1914.  In  1916  it  was 
tried  experimentally  at  the  Massachusetts  Institute  of  Tech- 
nology, and  it  has  been  tested  subsequently  at  other  places,  nota- 
bly at  Ne.w  Haven,  Conn.,  in  1917  and  1918.  It  is  one  of  the 
most  recent  developments  in  sewage  treatment  and  no  extensive 
experience  has  been  had  with  it.  The  process  consists  in  the 
acidification  of  sewage  with  sulphuric  or  sulphurous  acid,  as  the 
result  of  which  the  suspended  matter  and  grease  are  precipitated 
and  bacteria  are  removed.  The  equipment  required  for  the 
process  consists  of  devices  for  the  production  of  sulphur  dioxide 
(802),  and  for  feeding  niter  cake  or  other  forms  of  acid;  sub- 
siding basins;  sludge-handling  apparatus;  sludge  driers;  grease 
extractors;  grease  stills;  and  tankage  driers  and  grinders. 

The  first  step  is  the  acidification  of  the  sewage.  The  period 
of  contact  with  the  acid  is  about  4  hours.  Sulphurous  acid 
seems  to  give  better  results  than  sulphuric  because  of  the  ease 
in  which  it  can  be  manufactured  on  the  spot.  It  seems  also  to 
be  more  virulent  in  attacking  bacteria  than  an  equal  strength 
of  sulphuric  acid.  Jn  experimental  plants  the  acidulation  has 
been  accomplished  in  different  ways  such  as:  by  the  addition 
of  compressed  sulphur  dioxide  from  tanks;  by  the  addition  of 
sulphur  dioxide  made  from  burning  sulphur;  or  by  the  roasting 
of  iron  pyrite  (FeS2).  The  acidulation  precipitates  most  of  the 
grease  as  well  as  the  suspended  matter  and  results  in  a  sludge 
which  gives  some  promise  of  commercial  value.  In  referring 
to  the  process  R.  S.  Weston  states:1 

1  Reference  1,  at  end  of  this  chapter. 
482 


THE  MILES  ACID  PROCESS  483 

(1)  It  disinfects  the  sewage  by  reducing  the  numbers 
of  bacteria  from  millions  to  hundreds  per  c.c. 

(2)  If  the  drying  of  the  sludge  and  the  extraction  of 
the  grease  can  be  accomplished  economically,  it  is  possible 
that  a  large  part,  if  not  all,  of  the  cost  of  the  acid  treat- 
ment may  be  met  by  the  sale  of  the  grease  and  fertilizer 
recovered  from  the  sewage. 

(3)  The  use  of  so  strong  a  deodorizer  and  disinfectant 
as  sulphur  dioxide  would  prevent  the  usual  nuisances  of 
treatment  works. 

(4)  The   addition   of   sulphur   dioxide  to   the   sewage 
also  avoids  any  fly  nuisance,  which  is  a  handicap  to  the 
operation  of  Imhoff  tanks  and  trickling  filters. 

The  amount  of  acid  used  varies  with  the  quality  of  the 
sewage  and  the  desired  character  of  the  effluent.  At  Bradford, 
England,1  5,500  pounds  of  sulphuric  acid  are  used  per  million 
gallons,  producing  about  2,340  pounds  of  grease  or  0.43  pound  of 
grease  per  pound  of  sulphuric  acid.  At  Boston  only  0.215  pound 
of  grease  were  produced  per  pound  of  sulphuric  acid.  The  dif- 
ference is  probably  due  to  the  great  difference  in  the  amount  of 
grease  in  the  raw  sewage.  In  the  East  Street  sewer  at  New  Haven, 
Conn.,2  only  700  pounds  of  acid  are  used  per  million  gallons  of 
sewage  as  the  alkalinity  is  only  50  p. p.m.  This  amount  of  acid 
secures  an  acidity  of  50  p.p.m.  whereas  in  the  Boulevard  sewer 
1,130  pounds  of  acid  had  to  be  added  to  produce  the  same  result. 
The  results  obtained  by  the  experiments  conducted  by  the 
Massachusetts  State  Board  of  Health  in  1917  are  shown  in 
Table  97.  The  character  of  the  sludge  from  the  same  tests 
is  shown  in  Table  98.  After  acidification3  the  sewage  contains 
bisulphites  and  some  free  sulphurous  acid,  with  some  lime  and 
magnesium  soaps  which  are  attacked  by  the  acid  liberating  the 
free  fatty  acids.  Part  of  the  bisulphites  and  sulphurous  acid 
are  oxidized  to  bisulphates  and  sulphuric  acid.  It  was  found 
as  a  result  of  the  New  Haven3  experiments  that  the  presence 
of  sulphur  dioxide  in  the  effluent  caused  an  abnormal  oxygen 
demand  from  the  diluting  water  and  that  this  difficulty  could  be 
partly  overcome  by  the  aeration  of  the  effluent  after  acidulation 
and  sedimentation,  without  prohibitory  expense.  The  effluent 
and  sludge  are  both  stable  for  appreciable  periods  of  time  and 
are  suitable  for  disposal  by  dilution.  The  character  of  the 

1  Reference  2.         2  Reference  6.         3  Reference  5. 


484        ACIDIFICATION,  ELECTROLYSIS,   DISINFECTION 


sludge  as  determined  by  the  New  Haven  tests l  is  shown  in  Table 
99. 

TABLE  97 

AVERAGE  ANALYSIS  OF  SEWAGE   ENTERING  BOSTON  HARBOR,  BEFORE  AND 
AFTER  TREATMENT,  JULY  17  TO  SEPTEMBER  27,  19i7 

(Eng.  News-Record,  Vol.  80,  p.  319) 


Sample 

Parts  per  Million 

Bacteria, 
Millions 

Ammonia 

Kjeldahl 
Nitrogen 

Chlor- 
ine 

Oxy- 
gen 
Con- 
sumed 

Free 

Albuminoid 

Total 

Total 

Diss. 

Total 

Diss. 

20° 

37° 

Paddock's  Island 

Raw  sewage 

14.0 
'12.2 

20.9 

3.3 
1.6 

5.2 

1.8 
1.1 

3.9 

6.8 
3.5 

10.0 

3.6 
2.2 

7.5 

134 

23.1 
15.4 

1.86 

units 
94 

4.15 

units 
91 

Settled  sewage  
Acidified   and   settled 
sewage  

Deer  Island 

Raw  sewage  

23.3 
21.1 

20.9 

8.2 
5.6 

5.2 

4.8 
3.9 

3.9 

16.8 
10.7 

10.0 

8.9 
7.3 

7.5 

3100 

87.3 
62.2 

2.63 

units 
147 

1.50 

units 
85 

Settled  sewage  
Acidified   and   settled 

Calf  Pasture 


Raw  sewage  

18.0 

4.5 

2.0 

9.7 

4.1 

3254 

41.2 

1.89 

0.98 

Settled  sewage  

19.1 

2.3 

1.4 

4.9 

3.3 

25.8 

Acidified   and   settled 

units 

units 

17  8 

2.4 

1  6 

4.9 

3.3 

277 

149 

The  success  of  the  Miles  Acid  Process  in  comparison  with  other 
processes  is  dependent  on  the  commercial  value  of  the  sludge 
produced.  The  New  Haven  experiments  indicate  that  16  to  21 
per  cent  of  the  grease  in  the  sludge  is  unsaponifiable  and  seri- 
ously impairs  the  value  of  the  process. 

1  Reference  6. 


THE  MILES  ACID  PROCESS 


485 


TABLE  98 

AVERAGE  AMOUNT  OF  SLUDGE  AND  FATS  OBTAINED  FROM  SEWAGE  ENTER- 
ING BOSTON  HARBOR  AFTER  EIGHTEEN  HOURS  SEDIMENTATION 
WITH  AND  WITHOUT  ACIDIFICATION 

(Eng.  News-Record,  Vol.  80,  p.  319) 


Paddock's  Island 

Deer  Island 

Calf  Pasture 

Sedimentation 

Sedimentation 

Sedimentation 

Plain 

Acidu- 
lated 

Plain 

Acidu- 
lated 

Plain 

Acidu- 
lated 

Pounds  of  SOa  used  per  million 
gallons  of  sewage  treated  .... 
Dry  sludge  per  million  gallons.  . 
Per  cent  Nitrogen  in  sludge.  .  .  . 
Per  cent  fats  in  sludge  

762 
3.10 
27.30 

818 
959 
3.38 
27.30 

1709 
3.57 
24.60 

1513 
1939 
3.45 
19.40 

1189 
1427 
2.83 
26.30 

1208 
3.18 
24.30 

TABLE  99 

CHARACTER  OF  MILES  ACID  SLUDGE  AT  NEW  HAVEN 
(Eng.  News-Record,  Vol.  81,  p.  1034) 


East  Stre 

;et  Sewer 

Boule- 
vard 
Sewer 

Length  of  run  in  days  

25 

24 

44 

70 

29 

Total  sewage  treated,  thou- 

sand gallons  

260 

239.4 

407.8 

602.2 

145.5 

Gallons  wet  sludge  per  mil- 

lion gallons  sewage  

3750 

4025 

3200 

2600 

5375 

Specific  gravity  

1.067 

1.048 

1.054 

1.061 

Per  cent  moisture  

86.6 

88 

86.3 

85.7 

92.5 

Pounds  of  dry  sludge  per 

million  gallons  sewage  

503 

483 

439 

368 

403 

Ether  extract,  per  cent  dry 

sludge 

23.7 

24.0 

29 

32.6 

30.9 

Ether  extract,  pounds  per 

million  gallons  .  

119 

116 

127 

120 

124 

Volatile  matter,  per  cent  dry 

sludge 

47.2 

51.2 

57.3 

63.8 

78.5 

Nitrogen,  per  cent  dry  sludge 

1.6 

1.6 

2.4 

2.0 

3.0 

486         ACIDIFICATION,  ELECTROLYSIS,  DISINFECTION 

The  conclusions  reached  as  a  result  of  the  New  Haven  experi- 
ments are:1 

Our  experience  with  New  Haven  sewage  lends  no 
color  to  the  hope  that  a  net  financial  profit  can  be  obtained 
by  the  use  of  the  Miles  Acid  Process,  except  with  sewage 
of  exceptionally  high  grease  content  and  low  alkalinity. 
They  do,  however,  suggest  that  for  communities  where 
clarification  and  disinfection  are  desirable — where  screening 
would  be  insufficient  and  nitrification  unnecessary — the 
process  of  acid  treatment  comes  fairly  into  competition 
with  the  other  processes  of  tank  treatment,  and  that  it 
is  particularly  suited  to  dealing  with  sewages  that  contain 
industrial  wastes,  and  to  use  in  localities  where  local 
nuisances  must  be  avoided  at  all  costs  and  where  sludge 
disposal  could  be  provided  for  only  with  difficulty. 

The  conclusions  reached  as  a  result  of  the  Chicago  experi- 
ments are:2 

The  results  on  hand  indicate  that  treatment  of  this 
sewage  with  acid  results  in  a  somewhat  greater  retention 
of  fat.  An  apparent  reduction  in  the  oxygen  demand 
over  that  resulting  from  plain  sedimentation,  while  remark- 
able, is  probably  not  real,  being  simply  due  to  a  retarda- 
tion of  decomposition  by  the  sterilization  of  the  bacteria 
present,  the  organic  matter  being  left  in  solution.  .  .  . 
However,  there  appears  the  added  cost  of  acid  treatment 
and  the  cost  of  recovery  of  the  grease,  as  well  as  the 
uncertainty  of  the  price  to  be  received  for  the  grease 

recovered. 

v 

The  cost  of  the  treatment  is  estimated  by  Dorr  to  be  $18  per 
million  gallons,  and  the  value  of  the  sludge  obtained  from  the 
Boston  sewage  as  $24  per  million  gallons,  giving  a  net  margin 
of  profit  of  $6  per  million  gallons.  At  New  Haven,  the  total 
return  is  estimated  at  $7.09  per  million  gallons.  Based  on  the 
production  of  sulphur  dioxide  by  burning  sulphur  (assumed  to 
cost  $36  per  long  ton)  and  on  drying  from  85  per  cent  to  10 
per  cent  moisture  with  coal  assumed  to  cost  $7.50  per  ton,  it 
appears  that  the  acid  treatment  of  sewage  should  be  materially 
cheaper  than  either  the  Imhoff  treatment  or  fine  screening  under 
the  local  conditions.  A  comparison  of  the  cost  of  the  treatment 
of  the  East  Street  and  the  Boulevard  sewage  at  New  Haven 

1  Reference  6.  2  Reference  8. 


THE  MILES  ACID  PROCESS 


487 


and  the  Calf  Pasture  sewage  in  Boston  is  given  in  Table  100. 
The  cost  of  construction  was  estimated  by  Dorr  and  Weston 
in  1919  as  greater  than  $15,000  per  million  gallons  of  sewage 
per  day  capacity. 

TABLE  100 

ESTIMATED  COST  OF  SEWAGE  TREATMENT  AT  NEW  HAVEN  AND    BOSTON 
BY  THREE  DIFFERENT  PROCESSES 

Cost  in  Dollars  per  Million  Gallons  Treated 
(Engineering  and  Contracting,  Vol.  51,  p.  510) 


Miles  Acid  Process 

Imhoff  Tank  and 
Chlorination 

Fine  Screens 
and  Chlorination 

East 

Boule- 

Calf 

East 

Boule- 

Calf 

East 

Boule- 

Street 

vard 

Pasture 

Street 

vard 

Pasture 

Street 

vard 

Tanks   and   Buildings 

Int.  and  Dep  

2.47 

2.47 

•2.47 

5.28 

4.44 



4.60 

4.60 

Acid  treatment     .    . 

6.93 

10  74 

18  65 

Drying  sludge  

2.09 

2.04 

10.34 

Degreasing  sludge  .... 

1.78 

1.91 

9.12 

Redrying  tankage.  .  .  . 

0.17 

0.17 

0.10 

Superintendence   

1.06 

2.65 

1.06 

0  46 

1   15 

0  47 

1  15 

Labor    on    tanks    and 

screens  

1.00 

1.00 

1.00 

1.20 

1.50 

1.42 

2.05 

Disposal  of  sludge  or 

screenings   

1  00 

1.00 

0  50 

0  50 

Chlorination    

4.05 

4.05 

4.05 

4  05 

Gross  cost    

15.50 

20.98 

42.75 

11.99 

12.14 

11.03 

12.35 

6  57 

10  66 

47  59 

Net  cost 

8  93 

10  32 

4  84 

11  99 

12   14 

11  03 

12  35 

I 

ELECTROLYTIC   TREATMENT 

276.  The  Process. — This  process  has  been  generally  unsuc- 
cessful in  the  treatment  of  sewage  and  has  grown  into  disrepute. 
In  the  words  of  the  editor  of  the  Engineering  News-Record: 1 

Thirty  years  of  experiments  and  demonstrations  with 
only  a  few  small  working  plants  built  and  most  of  them 
abandoned — such  in  epitome  is  the  record  of  the  electro- 
lytic process  of  sewage  treatment. 

It  is  probably  true  that  the  process  has  never  received  a  thorough 
and  exhaustive  test  on  a  large  scale,  but  the  small-scale  tests  have 

1  Reference  20. 


488         ACIDIFICATION,   ELECTROLYSIS,   DISINFECTION 

not  been  promising  of  good  results.  Among  the  most  extensive 
tests  have  been  those  at  Elmhurst,  Long  Island,1  Decatur,  111.,2 
and  Easton,  Pa.3 

Whatever  degree  of  popularity  the  method  has  possessed 
has  been  due  possibly  to  the  mystery  and  romance  of  "  elec- 
tricity "  and  to  the  personality  of  its  promoters.  The  process 
should,  nevertheless,  be  understood  by  the  engineer  in  order 
that  it  may  be  explained  satisfactorily  to  the  layman  interested 
in  its  adoption. 

In  this  process,  sometimes  called  the  direct-oxidation  process, 
all  grit  is  removed  and  the  sewage  is  passed  through  fine  screens 
before  entering  the  electrolytic  tank.  In  the  electrolytic  tank 
the  sewage  passes  in  thin  sheets  between  electrodes  and  an 
electric  current  is  discharged  through  it.  A  recent  develop- 
ment has  been  the  addition  of  lime  to  the  sewage  at  some  point 
in  its  passage  through  the  electrolytic  tank.  From  the  elec- 
trolytic tank  the  sewage  flows  to  a  sedimentation  tank,  where 
sludge  is  accumulated,  and  from  which  the  liquid  effluent  is 
finally  disposed  of. 

It  is  claimed  that  the  action  of  the  electricity  electrolyzes 
the  sewage,  releasing  chlorine,  which  acts  as  a  powerful  disin- 
fectant. The  constituents  of  the  sewage  are  oxidized  so  that 
the  dissolved  oxygen,  nitrates,  and  relative  stability  are  increased 
and  the  sludge  is  rendered  non-put rescible.  It  is  said  that  the 
addition  of  lime  increases  the  efficiency  of  sedimentation  and 
enhances  the  effect  of  the  electric  current.  The  results  obtained 
by  tests  at  Easton,  Pa.,  are  shown  in  Table  101.  It  will  be 
observed  from  this  table  that  the  combination  of  lime  and 
electricity  does  not  have  a  more  beneficial  effect  than  either  one 
of  them  alone.  The  amount  of  sludge  produced  by  the  com- 
bination is  about  the  same  as  by  chemical  precipitation  alone, 
but  the  character  of  the  sludge  produced  with  electricity  is  less 
putrescible.  The  cost  of  the  treatment  as  estimated  at  Elm- 
hurst  is  shown  in  Table  102. 

As  a  result  of  the  tests  at  Decatur,  comparing  lime  alone 
with  lime  and  electricity  together,  Dr.  Ed.  Bartow  stated: 

The  purification  by  treatment  with  lime  alone  was 
greater  than  that  obtained  in  several  of  the  individual 
samples  treated  with  lime  and  electricity. 

1  Reference  17.  2  Reference  19.  3  Reference  21. 


DISINFECTION  OF  SEWAGE 


489 


TABLE  101 

COMPARATIVE  RESULTS  OBTAINED  FROM  THE  TREATMENT  OF  SEWAGE  BY 
LIME  ALONE,  ELECTRICITY  ALONE,  AND  LIME  AND  ELECTRICITY  COMBINED 

(Creighton  and  Franklin,  Journal  of  the  Franklin  Institute,  August,  1919) 


Lime  and 

Electricity 

Lime  Alone 

Electricity  Alone 

Change, 
Parts 

Change, 
Per 

Change, 
Parts 

Change, 
Per 

Change, 
Parts 

Change, 
Per 

per 
Million 

Cent 

per 

Million 

Cent 

per 
Million 

Cent 

Chlorine  

+  1.2 

+  1.9 

+  12.3 

+  18.2 

+  1.6 

+2.2 

Nitrites  

+0.014 

+58.3 

-.005 

-10.0 

-0.01 

-20.0 

Nitrates  .  .  .  ;  

+0.13 

+23.6 

+  .005 

+0.8 

-0.15 

-20.0 

Ammonia  

-3.3 

-18.3 

+0.2 

+  1.3 

+0.9 

+6.6 

Albuminoid      am- 

monia 

-3.6 

-12.1 

-0.4 

-1.7 

-0.5 

—2.3 

Oxygen  demand.  .  . 

-13.0 

-20.5 

-7.7 

-8.9 

-6.5 

-10.0 

Dissolved  oxygen  .  . 

+  1.78 

+40.9 

-0.93 

-19.1 

+1.61 

+40.1 

Total  bacteria   at 

37°  (Thousands) 

-343 

-92.7 

-373 

-82.4 

-165 

-37.8 

Total   bacteria   at 

20°  (Thousands) 

-688 

-92.2 

-1074 

-90.1 

-635 

-70.0 

B.     Coli      (Thou- 

sands)   

-77.9 

-99.85 

-96.3 

-92  3 

-45 

-81.8 

Oxygen     absorbed 

in  5  days  .  ,  

-3.40 

-81.6 

-1.03 

-21. 

+1.24 

+31 

DISINFECTION 

277.  Disinfection  of  Sewage. — Sewage  is  disinfected  in  order 
to  protect  public  water  supplies,  shell  fish,  and  bathing  beaches; 
to  prevent  the  spread  of  disease;  to  keep  down  odors,  and  to 
delay  putrefaction.  Disinfection  is  the  treatment  of  sewage 
by  which  the  number  of  bacteria  is  greatly  reduced.  Steriliza- 
tion is  the  destruction  of  all  bacterial  life,  including  spores. 
Ordinarily  even  the  most  destructive  agents  do  not  accomplish 
complete  sterilization.  Chlorine  and  its  compounds  are  practi- 
cally the  only  substances  used  for  the  disinfection  of  sewage. 
The  lime  used  in  chemical  precipitation,  the  acid  used  in  the  Miles 


490        ACIDIFICATION,  ELECTROLYSIS,   DISINFECTION 


Acid  Process,  the  aeration  in  the  activated  sludge  process,  all 
serve  to  disinfect  sewage,  but  are  not  used  primarily  for  that 
purpose.  Copper  sulphate  has  been  used  as  an  algaecide  but 
never  on  a  large  scale  as  a  bactericide.1  Heat  has  been  suggested, 
but  its  high  cost  has  prevented  its  practical  application  to  the 
disinfection  of  sewage. 

TABLE  102 

COST  OF  ELECTROLYTIC  TREATMENT,  ELMHURST,  LONG  ISLAND,  AND 
EASTON,  PENNSYLVANIA 


Three 

One  Million  Gallon 

Million 

Gallon 

Item 

unit  at 

unit  at 

unit  at 

Easton, 

Elmhurst, 

Elmhurst, 

Dollars 

Dollars 

Dollars 

Hydra  ted  lime: 

Elmhurst,  1300  pounds  at  $7.90  ton.  j 
Easton,      3720  pounds  at  $6.75  ton.  j 

12.56 

5.14 

15.42 

Electric  power  electrolysis: 

Elmhurst,    85      kw-h.  at  4        cents  1 
Easton,      185  .  5  kw-h.  at  2  .  26  cents  j 

4.19 

3.40 

9.60 

Electric  power,  light  and  agitation: 

Elmhurst,    60  kw-h.  at  4         cents  1 
Easton,        6  .  25  kw-h  .  at  8  .  05  cents  J 

0.50 

2.40 

7.20 

Heating  

1  25 

Labor  and  supervision                  

15  00 

12  50 

15  00 

Maintenance,  repairs  and  supplies  

1.50 

1.00 

3.00 

Sludge  pressing  and  removal  

5  11 

15  33 

Total           

35.00 

29.55 

65  55 

Cost  per  million  gallons  

35.00 

29.55 

21.85 

The  action  which  takes  place  on  the  addition  to  sewage  of 
chlorine  or  its  compounds  is  not  well  understood.  The  idea  that 
the  bacteria  are  burned  up  with  "  nascent "  or  freshly  born 
oxygen,  has  been  exploded.2  Likewise  the  idea  that  the  toxic 
properties  of  chlorine  have  no  effect  has  not  been  borne  out  by 

1  Reference  24. 

2  Inorganic  Chemistry,  by  Alexander  Smith. 


DISINFECTION  OF  SEWAGE  491 

experiments.  It  has  been  demonstrated,  particularly  by  tests 
on  strong  tannery  wastes,  that  the  action  of  chlorine  gas  is  more 
effective  than  the  application  of  the  same  amount  of  chlorine 
in  the  form  of  hypochlorite.  All  that  we  are  certain  of  at  present 
is  that  the  greater  the  amount  of  chlorine  added  under  the  same 
conditions,  the  greater  the  bactericidal  effect. 

Chlorine  is  applied  either  in  the  form  of  a  bleaching  powder 
or  a  gas.  In  ordinary  commercial  bleach  (calcium  hypochlorite) 
the  available  chlorine  is  about  35  to  40  per  cent  by  weight.  In 
order  to  add  one  part  per  million  of  available  chlorine  to  sewage 
it  is  necessary  to  add  about  25  pounds  of  bleaching  powder  or 
8|  pounds  of  liquid  chlorine  per  million  gallons  of  sewage.  This 
can  be  computed  as  follows: 

The  molecular  weight  of  calcium  hypochlorite  is 
127.0.  This  reacts  to  produce  two  atoms  of  available 
chlorine  with  a  molecular  weight  of  70.9.  If  the  bleach- 
ing powder  were  pure  the  available  chlorine  would  there- 
fore represent  70.9-1-127,  or  56  per  cent  of  its  weight. 
Then  to  obtain  one  pound  of  chlorine  it  would  be  neces- 
sary to  have  1.79  pounds  of  pure  bleaching  powder. 
Since  1,000,000  gallons  of  water  weigh  approximately 
8,300,000  pounds,  in  order  to  apply  one  part  per  million  of 
chlorine  to  1,000,000  gallons  of  sewage  it  is  necessary  to 
apply  1.79X8.3  or  14.9  pounds  of  pure  bleaching  powder. 
Commercial  bleaching  powder  is  only  about  60  per  cent 
calcium  hypochlorite.  It  is  therefore  necessary  to  add 
14.9^-0.60  or  about  25  pounds  of  commercial  bleach. 

Since  liquid  chlorine  is  very  nearly  pure,  approxi- 
mately 8|  pounds  of  it  applied  to  1,000,000  gallons  of 
sewage  are  equivalent  to  a  dose  of  one  part  per  million. 

Commercial  bleaching  powder  is  a  dry  white  powder  which 
absorbs  moisture  slowly,  and  which  loses  its  strength  rapidly 
when  exposed  to  the  air.  It  is  packed  in  air-tight  sheet  iron 
containers,  which  should  be  opened  under  water,  or  emptied 
into  water  immediately  on  being  opened.  The  strength  of  the 
solution  should  be  from  i  to  1  per  cent.  The  rate  of  the  appli- 
cation of  the  solution  to  the  sewage  may  be  controlled  by  auto- 
matic feed  devices,  or  by  hand-controlled  devices. 

Commercial  liquid  chlorine  is  sold  in  heavy  cast  steel  con- 
tainers, which  hold  100  to  140  pounds  of  liquid  chlorine  under  a 
pressure  of  54  pounds  per  square  inch  at  zero  degrees  C.  or  121 
pounds  per  square  inch  at  20  degrees. 


492         ACIDIFICATION,  ELECTROLYSIS,   DISINFECTION 

The  amount  of  chlorine  used  is  dependent  on  the  character 
of  the  sewage  to  be  treated,  the  stage  of  decomposition  of  the 
organic  matter,  the  desired  degree  of  disinfection,  the  period  of 
contact,  and  the  temperature.  The  amount  of  chlorine  is 
expressed  in  parts  per  million  of  available  chlorine,  regardless  of 
the  form  in  which  the  chlorine  is  applied.  In  general  about  15 
to  20  parts  per  million  of  available  chlorine  with  30  minutes' 
contact  at  a  temperature  of  about  15°  C.  will  effect  an  apparent 
removal  of  99  per  cent  of  the  bacteria  from  the  raw  sewage. 
The  effect  is  only  apparent  because  many  of  the  bacteria  encased 
in  the  solid  matter  of  the  sewage  escape  the  effect  cf  the  chlo- 
rine, or  detection  in  the  bacterial  analysis.  Stronger  and  older 
sewages,  higher  temperatures,  and  shorter  periods  of  contact 
will  demand  more  chlorine  to  produce  the  same  results.  A 
septic  effluent  will  require  more  chlorine  than  a  raw  sewage 
because  of  the  greater  oxygen  demand  by  the  septic  sewage. 
The  results  of  experiments  on  disinfection  made  at  different 
testing  stations  have  shown  such  wide  variations  in  the  amount 
of  chlorine  necessary,  as  to  demonstrate  the  necessity  for  inde- 
pendent studies  of  any  particular  sewage  which  is  to  be  chlori- 
nated. For  instance,  at  Milwaukee  approximately  13  p.p.m. 
of  available  chlorine  applied  to  an  Imhoff  tank  effluent  effected 
a  99  per  cent  removal  of  bacteria,  whereas  the  same  result  was 
obtained  at  Lawrence,  Mass.,  on  crude  sewage  with  only  6.6 
p.p.m.  and  at  Marion,  Ohio,  only  9  per  cent  removal  of  bacteria 
was  obtained  by  the  addition  of  4,815  p.p.m.  to  crude  sewage. 
The  Ohio  and  Massachusetts  reports  show  irrational  variations 
among  themselves.  For  instance,  6.2  p.p.m.  applied  to  a  septic 
effluent  effected  88  per  cent  removal  whereas  in  another  case 
7.6  p.p.m.  effected  only  36  per  cent  removal.  At  Lawrence  in 
one  case  it  took  8.6  p.p.m.  to  remove  99  per  cent  from  a  sand 
filter  effluent,  but  only  6.3  p.p.m.  to  effect  the  same  result  in  the 
effluent  from  a  septic  tank.  The  most  consistent  results  are 
those  found  at  Milwaukee  which  show  a  steadily  increasing 
percentage  removal  with  increasing  amounts  of  chlorine. 

Some  time  after  sewage  has  received  its  dose  of  chlorine  the 
number  of  bacteria  may  be  greater  than  in  the  raw*  sewage. 
Such  bacteria  are  called  after-growths.  Certain  forms  of  bac- 
teria, particularly  the  pathogenic  or  body  temperature  types, 
are  most  susceptible  to  disinfecting  agents.  These  are  killed 


REFERENCES  493 

( 

off  and  leave  the  sewage  in  a  condition  more  favorable  to  the 
growth  of  more  resistant  forms  of  bacteria.  As  the  latter  are 
non-pathogenic  and  are  generally  aerobic  their  presence  is 
usually  more  beneficial  than  detrimental,  as  they  hasten  the  action 
of  self-purification, 

REFERENCES 

The  following  abbreviations  will  be  used:  E.G.  for  Engineering  and 
Contracting,  E.N.  for  Engineering  News,  E.R.  for  Engineering  Record, 
E.N.R.  for  Engineering  News-Record,  M.J.  for  Municipal  Journal,  p.  for 
page,  and  V.  for  volume. 

No. 

1.  Grease  and  Fertilizer  Base  for  Boston  Sewage,  by  Weston,  E.N.  V.  75, 

1916,  p.  913  and  Journal  American  Public  Health  Association,  April, 
1916. 

2.  Getting  Grease  and  Fertilizer  from  City  Sewage,  by  Allen.    E.N.  V.  75, 

1916,  p.  1005. 

3.  New  Haven  Tests  Five  Processes  of  Sewage  Treatment.     E.N.R.  V.  79, 

1917,  p.  829. 

4.  Recovery  of  Grease  and  Fertilizer  from  Sewage  Comes  to  the  Front. 

E.N.R.  V.  80,  1916,  p.  319. 

5.  Miles  Acid  Process  may  Require    Aeration    of  Effluent,  by  Mohlman. 

E.N.R.     V.  81,  1918,  p.  235. 

6.  Promising    Results   with    Miles   Acid    Process    in    New    Haven    Tests. 
*   E.N.R.     V.  81,  1918,  p.  1034. 

7.  Baltimore  Experiments  on  Grease  from  Sewage.     E.N.     V.  75,   1916, 

p.  1155. 

8.  Report  on  Industrial  Wastes  from  the  Stock  Yards  and  Packingtown 

in  Chicago  to  the  Trustees  of  the  Sanitary  District  of  Chicago,  1914, 
pp.  187-195. 

9.  The   Separation   of   Grease   from   Sewage,  by   Daniels  and   Rosenfeld. 

Cornell  Civil  Engineer.     V.  24,  p.  13. 

10.  The  Separation  of  Grease  from  Sewage  Sludge  with  Special  Reference 

to  Plants  and  Methods  Employed  at  Bradford  and  Oldham,  England, 
by  Allen.     E.G.     V.  40,  1913,  p.  611. 

11.  Acid   Treatment   of   Sewage,   by   Dorr  and   Weston.     Journal   Boston 

Society    of    Civil    Engineers,    April,   1919.     E.G.     V.  51,    1919,    p. 
510.  M.  J.  V.  46,  1919,  p.  365. 

12.  The  Miles  Acid  Process  for  Sewage  Disposal.     Metallurgical  and  Chemical 

Engineering,  V.  18,  p.  591. 

13.  Miles  Acid  Treatment  of  Sewage,  by  Winslow  and  Mohlman.     Journal 

American  Society  Municipal  Improvements,  Oct.,  1918.  M.  J.  V.    45, 

1918,  pp.  280,  297,  and  321. 

14.  New  Electrolytic  Sewage  Treatment.     M.J.    V.  37,  1914,  p.  556. 


494         ACIDIFICATION,  ELECTROLYSIS  DISINFECTION 

No. 

15.  Electrolytic  Sewage  Treatment.     M.J.    V.  47,  1919,  p.  131. 

16.  Electrolytic  Treatment  of  Sewage  at  Durant,  Oklahoma,  by  Benham. 

E.N.  V.  76,  1916,  p.  547.  Municipal  Engineering,  V.  49,  1916, 
p.  141. 

17.  Electrolytic  Treatment  of  Sewage  at  Elmhurst,  Long  Island,  by  Travis. 

Report  to  the  President  of  the  Borough  of  Queens,  Aug.  31,  1914. 
E.R.  V.  70,  1914,  pp.  292,  315,  and  429.  M.J.  V.  39,  p.  551.  Muni- 
cipal Engineering,  V.  47,  p.  281. 

18.  Tests  of  the  Electrolysis  of  Sewage  at  Toronto,  by  Nevitt.    E.N.  V.  71, 

1914,  p.  1076. 

19.  Electrolytic  Treatment  of  Sewage  Little  Better  than  Lime  Alone,  by 

Bartow.     E.R.  V.  74,  1916,  p.  596. 

20.  Electrolytic    Sewage    Treatment    Not    Yet    an    Established    Process. 

E.N.R.  V.  83,  1919,  p.  541. 

21.  Tests  of  Electrolytic  Sewage  Treatment  Process  at  Easton,  Pa.     Journal 

of  the  Franklin  Institute,  Aug.,  1919.     E.N.R.  V.  83,  1919,  p.  569. 

22.  The  Disinfection  of  Sewage.     U.  S.  Geological  Survey,  Water  Supply 

Paper,  No.  229. 

23.  Sewage  Disinfection  in  Actual  Practice,  by  Orchard.     E.R.  V.  70,  1914, 

p.  164. 

24.  Water  and  Sewage  Purification  in  Ohio.    Report  of  the  Ohio  State  Board 

of  Health,  1908,  pp.  738-762. 

25.  Water    Purification,  by   Ellms.     Published   in    1917   by    McGraw-Hill 

Book  Co. 

26.  Electrolytic   Sewage   Treatment,   A   Half   Century   of   Invention   and 

Promotion.    E.N.'R.  V.  86,  1921,  p.  25. 


CHAPTER  XX 
/ 

SLUDGE 

278.  Methods  of  Disposal. — Sludge  is  the  deposited  suspended 
matter  which  accumulates  as  the  result  of  the  sedimentation 
of  sewage.     The  methods  for  the  disposal  of  sludge  as  discussed 
herein  will  include  the  disposal  of  scum.     Scum  is  a  floating 
mass  of  sewage  solids  buoyed  up  in  part  by  entrained  gas  or 
grease,  forming  a  greasy  mat  which  remains  on  the  surface  of  the 
sewage.1     The  sludges  formed  by  different  methods  of  sewage 
treatment  are  described  in  the  chapter  devoted  to  the  particular 
method.     The  disposal  of  sludge  is  a  problem  common  to  all 
methods  of  sewage  treatment  involving  the  use  of  sedimentation 
tanks. 

Sludge  is  disposed  of  by:  dilution,  burial,  lagooning,  burning, 
filling  land,  and  as  a  fertilizer  or  fertilizer  base.  Certain  methods 
of  disposal,  such  as  burning  or  as  a  fertilizer,  demand  that  the 
sludge  be  dried  preparatory  to  disposal.  Sludge  is  dried  on  dry- 
ing beds,  in  a  centrifuge,  in  a  press,  in  a  hot-air  dryer,  or  by 
acid  precipitation. 

279.  Lagooning. — This   is   a   method   of   sludge   disposal   in 
which  fresh  sludge  is  run  on  to  previously  prepared  beds  to  a 
depth  of  12  to  18  inches  or  more,  and  allowed  to  stand  without 
further    attention.     The    preparation    of    the    lagoons    requires 
leveling   the   ground,    building    of    embankments,    and,    if    the 
ground  is  not  porous,  the  placing  of  underdrains  laid  in  sand 
or    gravel.    At    Reading,    Pa.,2    approximately    one    acre    was 
required  for  1,700  cubic  yards  of  wet  sludge.     The  results  of 
lagooning  at  Philadelphia  are  given  in  Table  103.2 

1  American  Public  Health  Association  definition. 

2  Sewage  Sludge  by  Allen. 

495 


496 


SLUDGE 


TABLE  103 

RESULTS  OF  DRYING  SLUDGE  IN  LAGOONS  AT  PHILADELPHIA 
("Sewage  Sludge"  by  Allen) 


Treatment 

Days 

Depth, 
Inches 

Per  Cent, 
Moisture 

Rainfall, 
Inches 

Cubic  Yards 
per  Acre 

Screened  

0 

12.20 

82.8 

0 

1600 

Screened  

26 

7.67 

57  0 

0 

1000 

Screened 

49 

3  50 

51  6 

0  43 

470 

Screened 

o 

13  50 

90  1 

o 

1800 

Screened  

62 

7.00 

61  0 

3  14 

950 

Crude 

0 

12  00 

88  7 

0 

1600 

Crude  

59 

4.70 

62.8 

2.59 

640 

During  the  period  of  standing  in  the  lagoon  the  moisture 
drains  out  and  evaporates  and  the  organic  matter  putrefies, 
giving  off  gases  and  foul  odors.  In  the  course  of  three  to  six  months, 
biological  action  ceases  and  the  sludge  has  become  humified  and 
reduced  to  about  75  per  cent  moisture.  In  the  utilization  of 
this  method  of  disposal  the  lagoons  must  be  removed  from 
settled  districts  and  should  occupy  land  of  little  value  for  other 
purposes.  The  odors  created  at  the  lagoons  may  be  intense 
and  offensive.  The  land  so  used  is  rendered  unfit  for  other  pur- 
poses for  many  years. 

The  digestion  of  sludge  in  special  tanks  is  a  form  of  lagooning 
in  which  an  attempt  is  made  to  maintain  septic  action  as  a  result 
of  which  a  portion  of  the  sludge  is  gasified  or  liquefied,  leaving 
less  to  be  cared  for  by  some  of  the  other  methods  of  treatment 
or  disposal.  The  results  obtained  by  digestion  tanks  has  not 
been  entirely  satisfactory.  A  partial  drying  and  consolidation 
of  the  sludge  may  be  effected,  however,  by  the  process  of  decanta- 
tion,  in  which  the  supernatant  liquid  is  run  off,  followed  by  further 
sedimentation,  rendering  the  final  product  more  compact. 

280.  Dilution. — In  the  disposal  of  sludge  by  dilution,  as  in 
the  disposal  of  sewage  by  dilution,  there  must  be  sufficient 
oxygen  available  in  the  diluting  water  to  prevent  putrefaction, 
and  a  swift  current  to  prevent  sedimentation.  Such  conditions 
exist  in  localities  along  the  sea  coast,  and  in  communities 


DILUTION  497 

situated  near  rivers,  when  the  rivers  are  in  flood.  In  some  sea- 
coast  towns,  for  example  at  London  and  Glasgow,  the  sludge  is 
taken  out  to  sea  in  boats,  and  dumped.  Since  it  is  not  necessary 
to  discharge  sludge  continuously,  it  can  be  stored  to  advantage 
in  the  digestion  chamber  of  a  tank,  until  the  conditions  in  the 
body  of  diluting  water  are  suitable  to  receive  it. 

The  amount  of  diluting  water  to  receive  sewage  sludge  has  not 
been  sufficiently  well  determined  to  draw  reliable  general  con- 
clusions. A  dilution  of  1,500  to  2,000  volumes  may  be  considered 
sufficiently  safe  to  avoid  a  nuisance  provided  there  is  a  sufficient 
velocity  to  prevent  sedimentation.  Johnson's  Report  on  Sew- 
age Purification  at  Columbus,  Ohio  (1905),  states  that  a  dilution 
of  1  to  800  is  sufficient  to  avoid  a  nuisance.  The  character 
of  the  sludge  has  a  marked  effect  on  the  proper  ratio  of  dilution, 
the  sludge  from  septic  and  sedimentation  tanks  requiring  a 
greater  dilution  than  that  from  Imhoff  tanks. 

281.  Burial. — Sludge  can  be  disposed  of  by  burial  in  trenches 
about  24  inches  deep  with  at  least   12  inches  of  earth  cover, 
without  causing  a  nuisance.     The  ground  used  for  this  purpose 
should  be  well  drained.     This  method  of  disposal  is  generally 
used  as  a  makeshift   and  has  not  been   practiced  extensively 
because  of  the  large  amount  of  land  required.     Insufficient  infor- 
mation is  available  to  generalize  on  the  amount  of  land  required 
or  the  time  before  the  land  can  be  used  for  further  sludge  burial, 
or  for  other   purposes.     Indications   are  that   the   sludge  may 
remain  moist  and  malodorous  for  years  and  that  the  land  may  be 
rendered   permanently  unfit  for  further  sludge  burial.     Under 
some  conditions  the  land  may  be  used  again  for  the  same  or 
other  purposes.     For  example,  Kinnicutt,  Winslow  and  Pratt 1 
state  that  500  tons  of  wet  sludge  can  be  applied  per  acre  and : 

The  same  land,  it  is  claimed,  can  be  used  again  after 
a  period  of  a  year  and  a  half  to  two  years,  if  in  two  months 
or  so  after  covering  the  sludge  with  earth,  the  ground  is 
broken  up,  planted,  and,  when  the  crop  is  removed,  again 
plowed  and  allowed  to  remain  fallow  for  about  a  year. 

282.  Drying. — Before  sludge  can  be  disposed  of  to  fill  land, 
by  burning,  or  for  use  as  a  fertilizer  filler  it  must  be  dried  to  a 
suitable  degree  of  moisture.     The  removal  of  moisture  from  the 

1  Sewage  Disposal  by  Kinnicutt,  Winslow  and  Pratt. 


498  SLUDGE 

sludge  decreases  its  volume  and  changes  its  characteristics  so 
that  sludge  containing  75  per  cent  moisture  has  lost  all  the  char- 
acteristics of  a  liquid.  It  can  be  moved  with  a  shovel  or  fork, 
and  can  be  transported  in  non-watertight  containers.  A  reduction 
in  moisture  from  95  to  90  per  cent  will  cut  the  volume  in  half. 

The  change  in  volume  on  the  removal  of  moisture  can  be 
represented  as: 

_  7(100-  P) 
~  (100- Pi)7 

in  which  P  =  the  original  percentage  of  moisture; 
PI  =  the  final  percentage  of  moisture; 
F=the  original  volume; 
Vi  =  the  final  volume. 

The  drying  of  sludge  on  coarse  sand  filter  beds  is  more 
particularly  suited  to  sludge  from  Imhoff  tanks.  This  sludge 
does  not  decompose  during  drying,  and  is  sufficiently  light  and 
porous  in  texture  to  permit  of  thorough  draining.  The  sludge 
from  plain  sedimentation  or  chemical  precipitation  tanks  is 
high  in  moisture,  putrescible,  and  when  placed  on  a  filter  bed 
it  settles  into  a  heavy,  compact,  impervious  mass  which  dries 
slowly.  In  order  to  avoid  this  condition  the  sludge  is  run  on  to 
the  beds  as  quickly  as  possible,  to  a  depth  of  not  more  than  6 
to  10  inches.  Lime  is  sometimes  added  to  the  sludge  at  this 
time  as  it  aids  drying  by  assisting  in  the  maintenance  of  the 
porosity  of  the  sludge,  and  it  is  advantageous  in  keeping  down 
odors  and  insects. 

Sludge  filter  beds  are  made  up  of  12  to  24  inches  of  coarse 
sand,  well-screened  cinders,  or  other  gritty  material,  underlaid 
by  6  inches  of  coarse  gravel  and  6  or  8-inch  open-joint  tile 
underdrains,  laid  4  to  10  feet  apart  on  centers,  dependent  on  the 
porosity  of  the  subsoil.  The  side  walls  of  the  filters  are  made  of 
planks  or  of  low  earth  embankments.  The  sludge  filters  at 
Hamilton,  Ontario,  are  shown  in  Fig.  179. 

The  size  of  the  bed  is  dependent  mainly  upon  the  character- 
istics of  the  sludge.  For  Imhoff  tank  sludge  which  comes  from 
the  tank  with  about  85  per  cent  moisture,  the  practice  is  to 
allow  about  350  1  square  feet  of  filter  surface  per  1,000  popu- 
1  Sewage  Disposal  by  Fuller. 


DRYING 


499 


lation  contributing  sludge.  For  other  types  of  sludge  the  area 
varies  from  900  to  9,000  square  foot  per  1,000  population  con- 
tributing sludge,  and  only  experiments  with  the  sludge  in  hand 
can  determine  the  proper  allowance.  Imhoff  recommends  1,080 


^8" C.I. Sludge  Pipe, 
Part  Plan, 


r« 


Part  Cross  Section  A-B, 


FIG.  179. — Sludge-drying  Beds  at  Hamilton,  Ontario. 
Eng.  News,  Vol.  73,  p.  426, 

square  feet  per  1,000  population  for  septic  tank  sludge,  and 
6,480  square  feet  for  sludge  from  plain  sedimentation  tanks.1 
Kinnicutt,  Winslow,  and  Pratt  in  their  book  on  Sewage  Disposal 
state : 

With  an  average  depth  of  10  inches  per  dose  of  sludge 
of  87  per  cent  water  content,  one  square  foot  of  covered 
(glass)  bed  should  dry  to  a  spadable  condition  one  cubic 
yard  of  sludge  per  year. 


Sewage  Sludge  by  Allen. 


500  SLUDGE 

The  sludge  is  run  on  the  bed  in  small  quantities  at  periods  from 
two  weeks  to  a  month  apart.  In  favorable  weather  Imhoff 
sludge  will  dry  in  two  weeks  or  less  to  approximately  50  to  60 
per  cent  moisture.  It  is  then  suitable  for  use  as  a  filling  material 
on  waste  land,  for  burning,  or  for  further  drying  by  heat.  Glass 
roofs,  similar  to  those  used  on  green-houses,  have  been  used  to 
speed  the  drying  process  by  preventing  the  moistening  of  partly 
dried  sludge  during  rainy  weather.  In  some  instances  sludge 
has  dried  to  10  per  cent  moisture  on  such  beds.  Imhoff  sludge 
can  be  removed  from  the  drying  beds  with  a  manure  or  hay 
fork.  It  has  an  odor  similar  to  well-fertilized  garden  soil.  It 
is  stable,  dark  brownish-gray  in  color,  is  of  light  coarse  material, 
and  is  granular  in  texture. 

Sludge  presses  are  suitable  for  removing  moisture  from  the 
bulky  wet  sludge  obtained  from  plain  sedimentation,  chemical 
precipitation,  and  the  activated  sludge  process.  The  details 
of  a  typical  sludge  press  are  shown  in  Fig.  180.  The  press 
shown  is  made  up  of  a  number  of  corrugated  metal  plates  about 
30  inches  in  diameter  with  a  hole  in  the  center  about  8  inches 
in  diameter.  The  corrugations  run  vertically  except  for  a  distance 
about  3  inches  wide  around  the  outer  rim,  which  is  smooth.  To 
this  smooth  portion  is  fastened,  on  each  side  of  the  plate,  an 
annular  ring  about  an  inch  thick  and  2  to  3  inches  wide, 
of  the  same  outside  diameter  as  the  plate.  A  circular  piece 
of  burlap,  canvas,  or  other  heavy  cloth  is  fastened  to  this 
ring,  covering  the  plate  completely.  A  hole  is  cut  in  the  center 
of  the  cloth  slightly  smaller  in  diameter  than  the  center  hole 
in  the  plate,  and  the  edges  of  the  cloth  on  opposite  sides  of 
the  plate  are  sewed  together.  The  plates  are  then  pressed 
tightly  together  by  means  of  the  screw  motion  at  the  left  end 
of  the  machine,  thus  making  a  water-tight  joint  at  the  outer 
rim.  Sludge  is  then  forced  under  pressure  into  the  space 
between  the  plates,  passing  through  the  machine  by  means  of 
the  central  hole.  The  pressure  ori  the  sludge  may  be  from  50 
to  100  pounds  per  square  inch.  This  pressure  forces  the  water 
out  of  the  sludge  through  the  porous  cloth  from  which  it  escapes 
to  the  bottom  of  the  press  along  the  corrugations  of  the  sepa- 
rating plate.  After  a  period  of  10  to  30  minutes  the  pressure 
is  released,  the  cells  are  opened,  and  the  moist  sludge  cake  is 


DRYING 


501 


removed.  The  liquid  pressed  from  the  sludge  is  highly  putres- 
cible  and  should  be  returned  to  the  influent  of  the  treatment 
plant.  The  pressing  of  wet  greasy  sludges  is  facilitated  by  the 
addition  of  from  8  to  10  pounds  of  lime  per  cubic  yard  of  sludge. 
The  cake  thus  formed  is  more  cohesive  and  easy  to  handle.  The 
output  of  the  press  depends  so  much  on  the  character  of  the 
sludge  that  a  definite  guarantee  of  capacity  is  seldom  given  by 
the  manufacturer. 

The  simplest  form  of  centrifugal  sludge  dryer  is  a  machine 
which  consists  of  a  perforated  metal  bowl  lined  with  porous 
cloth  in  which  the  sludge  is  placed.  Surrounding  this  bowl  is 


FIG.  180.— Filter  Press. 

a  second  water-tight  metal  bowl  so  arranged  as  to  intercept  the 
water  thrown  from  the  sides  of  the  inner  bowl  as  it  revolves. 
The  peripheral  velocity  of  the  inner  bowl  is  about  6,000  feet  per 
minute,  which  makes  the  effective  weight,  of  each  particle  about 
250  times  its  normal  weight  when  at  rest.  Very  few  data  are 
available  on  the  operation  of  such  machines,  and  their  use  has 
not  been  extensive  because  of  the  difficulty  of  starting  and 
stopping  the  machine  at  each  filling,  and  the  difficulty  of  removing 
the  partially  dried  sludge  from  the  inner  basket.  The  Besco- 
ter-Meer  centrifuge,  manufactured  by  the  Earth  Engineering 
and  Sanitation  Co.,  can  be  operated  continuously  and  the  diffi- 
culties of  removing  the  dried  sludge  from  the  machine  have 


502 


SLUDGE 


been  overcome.  According  to  the  manufacturers  the  centri- 
fuge has  been  operated  very  successfully  in  Germany  on  plain 
septic  tank  sludge.  A  removal  of  70  per  cent  of  suspended  solids 
in  the  raw  sludge  and  a  production  of  3,600  pounds  of  sludge 
per  hour,  containing  60  to  70  per  cent  of  moisture,  can  be  obtained 
at  less  than  900  r.p.m.  with  a  consumption  of  15  horse-power. 
Extensive  tests  of  the  machine  were  made  at  Milwaukee 
from  October,  1920,  to  September,  1921,  on  activated  sludge, 


Besco-ter-Meer  Sludge  Drying  Centrifuge  at  Milwaukee,  Wisconsin 
Courtesy,  Barth  Engineering  and  Sanitation  Co. 

but  results  of  these  tests  are  not  as  yet  available.  Indications 
are  that  the  centrifuge  has  acted  as  a  classifier.  The  coarser 
particles  of  sludge  have  been  removed  but  the  finer  particles 
have  been  continuously  returned  with  the  liquid  to  the 
sedimentation  tank,  ultimately  filling  this  tank  with  fine 
particles  of  sludge.  An  illustration  of  the  unit  tested  at  Mil- 
waukee is  shown  on  this  page. 


DRYING 


503 


Experiments  on  the  drying  of  sludge  by  acid  flotation  have 
not  progressed  sufficiently  to  allow  the  installation  of  a  working 
unit.  The  method,  which  has  been  applied  principally  to 
activated  sludge,  consists  in  adding  a  small  amount  of  sulphuric 
acid  to  the  sludge  as  it  leaves  the  storage  tank.  The  sludge  is 
coagulated  by  this  action,  the  coagulated  material  rising  to  the 
surface  as  a  scum  containing  about  86  per  cent  moisture.  The 
consistency  is  such  that  it  can  be  removed  with  a  shovel.  The 
liquid  can  be  withdrawn  continuously  from  below  the  scum. 

The  moisture  content  of  sludge  to  be  used  in  the  manufacture 
of  fertilizer  must  be  reduced  to  10  per  cent  or  less.  None  of 


FIG.  181. — Direct-Indirect  Sludge  Dryer. 

Courtesy,  the  Buckeye  Dryer  Co. 

the  methods  of  drying  described  so  far  can  be  relied  upon  for 
such  a  product  and  it  becomes  necessary  to  use  direct  or  indirect 
heat  dryers.  There  are  various  types  of  dryers  on  the  market. 
The  details  of  a  Buckeye  dryer  are  shown  in  Fig.  181.  In  the 
operation  of  this  machine  moist  sludge  is  fed  in  at  the  left  end 
at  the  point  marked  "feed."  The  hot  gases  pass  from  the  fire  box 
up  and  around  the  cylinder  which  revolves  at  about  eight  r.p.m. 
The  gases  are  drawn  into  the  inner  cylinder  through  the  open- 
ings marked  A  which  revolve  with  the  two  cylinders.  The  gases 
escape  from  the  inner  cylinder  through  the  openings  to  the 
right  and  flow  towards  the  left  in  the  outer  cylinder,  They  come 


504  SLUDGE 

in  contact  with  the  sludge  at  this  point.  The  gases  then  pass 
off  through  the  fan  at  the  left.  The  sludge  is  lifted  by  the  small 
longitudinal  baffles  fastened  to  the  outer  cylinder,  as  the  drying 
cylinders  revolve.  The  right  end  of  the  cylinder  is  placed  lower 
than  the  left  so  that  the  drying  sludge  is  lifted  and  dropped 
through  the  cylinder  at  the  same  time  that  it  moves  slowly 
toward  the  right-hand  end  of  the  cylinder.  These  dryers 
require  about  one  pound  of  fuel  for  10  pounds  of  water  evaporated. 
The  odors  from  the  dryer  can  be  suppressed  by  passing  the  gases 
through  a  dust  chamber  and  washer. 

A  summary  of  the  results  from  methods  of  sludge  drying 
at  Milwaukee  1  follows: 

Excess  sludge  produced,  12,100  gallons,  having  97.5 
per  cent  moisture,  per  million  gallons  of  sewage  treated. 

Sludge  cake  produced  (by  presses),  10,083  pounds 
having.  80.3  per  cent  moisture,  per  million  gallons  of 
sewage  treated. 

Dried  sludge  (from  heat  driers)  produced,  2,521  pounds 
having  10  per  cent  moisture,  per  million  gallons  of  sewage 
treated. 

Press  will  produce  3  pounds  of  cake  per  square  foot  of 
filter  cloth  in  four  and  a  half  hours,  or  five  operations  per 
twenty-four  hours. 

Dryers  will  reduce  6,700  pounds  of  sludge  cake  at 
80  per  cent  moisture  to  10  per  cent  moisture,  and  will 
evaporate  8  pounds  of  water  per  pound  of  combustible. 

x    * 

Thickening  devices  known  as  Dorr  thickeners,  patented  and 
manufactured  by  the  Dorr  Co.  and  originally  intended  for 
metallurgical  purposes,  have  been  adapted  to  the  thickening 
of  sewage  sludge.  These  thickeners  are  circular  sedimentation 
tanks,  from  8  to  12  feet  deep,  more  or  less,  and  are  made  in  any 
diameter  up  to  200  feet  or  more.  An  arm,  pivoted  in  the  center 
and  extending  to  the  circumference,  is  provided  at  the  bottom 
with  a  number  of  baffles  or  squeegees  set  at  an  angle  with  the 
arm.  The  arm  revolves  at  from  one  to  fifteen  revolutions  per 
hour,  and  the  squeegees,  in  contact  with  the  bottom  of  the  tank, 
scrape  the  deposited  sludge  towards  a  central  sump,  from  which 

1  From  Eng.  News-Record,  Vol.  84,  1920,  p.  995. 


DRYING  505 

/ 

it  is  removed  by  a  pump  or  by  gravity,  without  interrupting 
the  operation  of  the  thickener.  The  sludge  so  thickened  may  be 
reduced  to  95  or  96  per  cent  moisture.  These  devices  are  ordi- 
narily used  only  in  the  activated  sludge  process  in  which  they 
have  been  a  pronounced  success. 


CHAPTER  XXI 
AUTOMATIC  DOSING  DEVICES 

283.  Types. — Automatic  dosing  devices  are  used  to  apply 
sewage  to  contact  beds,  trickling  filters,  and  intermittent  sand 
filters.     These  devices  can  be  separated  into  two  classes;    those 
with  moving  parts  and  those  without  moving  parts.     The  latter 
are  better  known  as  air-locked  dosing  devices.     Simple  devices 
without  moving  parts  are  less  liable  to  disorders  and  are  nearer 
"  fool-proof  "  than  any  device  depending  on  moving  parts  for 
its  operation. 

No  one  type  of  moving  part  device  has  been  used  extensively 
in  different  sewage  treatment  plants.  Designing  engineers  have 
exercised  their  ingenuity  at  different  plants,  resulting  in  the 
production  of  different  types.1  Among  the  best  known  forms 
is  the  apparatus  designed  by  J.  W.  Alvord  for  the  intermittent 
sand  filters  at  Lake  Forest,  Illinois.2  In  its  operation.  ,  ,  , 

A  float  in  the  dosing  chamber  lifts  an  iron  ball  in  one 
of  a  series  of  wooden  columns,  and  at  a  certain  height 
the  ball  rolls  through  a  trough  from  one  column  to  the 
next,  in  its  passage  striking  a  catch,  which  opens  an  air 
valve  attached  to  one  of  ten  bell-siphons  in  the  dosing 
chamber.  Each  of  the  siphons  discharges  on  one  of  the 
ten  sand  beds,  which  are  thus  dosed  in  rotation, 

Since  air-locked  dosing  devices  are  in  more  general  use  their 
operation  will  be  explained  in  greater  detail. 

284.  Operation. — The  simplest  form  of  these  devices  is  the 
automatic  siphon   used  for  flush-tanks,  the  operation  of  which 
is  described  in  Art.  61. 

In  the  operation  of  sand  filters,  sprinkling  filters,  or  other 
forms  of  treatment  where  there  are  two  or  more  units  to  be  dosed 

1  A  Simple  Mechanical  Control  for  Dosing  Sewage  Beds,  by  P.  Thompson, 
Eng.  News-Record,  Vol.  84,  1920,  p.  1018. 

2  Sewage  Disposal  by  Kinnicutt,  Winslow  and  Pratt. 

506 


OPERATION 


507 


it  is  desirable  that  the  dosing  of  the  beds  be  done  alternately. 
A  simple  arrangement  for  two  siphons  operating  alternately  is 
shown  in  Fig.  182.  They  operate  as  follows:  with  the  dosing 
tank  empty  at  the  start  water  will  stand  at  W  in  siphon  No.  2 
and  at  aa'  in  siphon  No.  1.  As  the  water  enters  through  the 
inlet  on  the  left  the  tank  fills.  When  the  water  rises  sufficiently, 
air  is  trapped  in  the  bells,  and  as  the  water  continues  to  rise  in 
the  tank,  surfaces  a  and  b  are  depressed  an  equal  amount.  When 
b  has  been  depressed  to  d,  a  has  been  depressed  to  c.  Air  is 
released  from  siphon  No.  2  through  the  short  leg,  and  siphon 


Inlet 


No.  I 


No.. 2 


Plan, 


:               * 

v                 : 

!    1(11 

ff"s]    ^ 

i     if 

si 

§     ; 

i   i 

\    \ 

^ 

'         1  I)- 

m 

o 

Q 

V%^5/7 

*^? 

Section 

.  ^ 

A-A 

VV^ 

•  Dosing 
Tank 


Pipe 


FIG.  182. — Diagram  Showing  the  Operation  of  Two  Alternating  Siphons. 

No.  2  goes  into  operation.  Surface  c  rises  in  siphon  No.  1  as 
the  tank  empties  and  when  the  action  of  Siphon  No.  2  is  broken 
by  the  admission  of  air  when  the  bottom  of  the  bell  is  uncovered 
the  water  in  siphon  No.  1  has  assumed  the  position  of  W  and  that 
in  No.  2  is  at  aa'.  The  conditions  of  the  two  siphons  are  now 
reversed  from  that  at  the  beginning  of  the  operation  and  as  the 
tank  refills  siphon  No.  1  will  go  into  operation.  It  is  to  be 
noted  that  these  siphons  are  made  to  alternate  by  weakening 
the  seal  of  the  next  one  to  discharge  and  by  strengthening  the 
seal  of  the  one  which  has  just  discharged. 


508 


AUTOMATIC  DOSING  DEVICES 


285.  Three  Alternating  Siphons.— This  principle  can  be 
extended  to  the  operation  of  three  alternating  siphons  as  shown 
in  Fig.  No.  183.  These  operate  as  follows:  with  the  dosing 
tank  empty  at  the  start  and  water  at  aar  in  siphons  1  and  2, 
and  at  bb'  in  siphon  No.  3,  the  dosing  tank  will  be  allowed  to 
fill.  As  the  water  rises  in  the  tank  air  is  trapped  in  all  the  bells 
and  surfaces  a  and  b  are  depressed.  When  surface  6  has  been 
depressed  to  d,  a  has  been  depressed  to  c.  Air  is  released  from 
siphon  No.  3  and  this  siphon  goes  into  action.  Surface  c  rises 


Inlet 


No.  I 


No,? 


Plan 


FIG.  183. — Diagram  Showing  the  Operation  of  Three  Alternating  Siphons. 

in  siphons  1  and  2  to  the  position  6,  as  the  dosing  tank  is  emptied. 
At  the  same  time  a  small  amount  of  water  is  passed  from  siphon 
No.  3  to  the  short  leg  of  siphon  No.  1,  through  the  small  pipes 
shown,  thus  filling  this  leg  so  that  when  siphon  No.  3  ceases  to 
operate  the  water  in  siphons  1  and  3  stands  at  aa'  and  that  in 
No.  2  stands  at  bb'.  Siphon  No.  2,  having  the  weaker  seal, 
will  be  the  next  to  operate.  During  its  operation  it  will  fill 
siphon  No.  3,  leaving  No.  1  weak.  When  No.  1  operates  it  will 
refill  No.  2,  leaving  No.  3  weak,  thus  completing  a  cycle  for  the 
three  siphons.  This  principle  has  not  been  applied  to  the  operation 


FOUR  OR  MORE  ALTERNATING  SIPHONS 


509 


of  more  than  three  alternating  siphons  and  is  seldom  used  on  recent 
installations. 

286.  Four  or  More  Alternating  Siphons. — An  arrangement 
for  the  alternation  of  four  or  more  siphons  is  illustrated  in  Fig.  184. 
At  the  commencement  of  the  cycle  it  will  be  assumed  that  all 
starting  wells  are  filled  with  water  except  well  No.  1,  and  that  all 
main  and  all  blow-off  traps  are  filled  with  water.  The  following 


FIG.  184. — Miller  Plural  Alternating  Siphons. 
Courtesy,  Pacific  Flush  Tank  Co. 

description   of  the  operation  of  the  siphons  is  taken  from  the 
catalog  of  the  Pacific  Flush  Tank  Company : 

The  liquid  in  the  tank  gradually  rises  and  finally 
overflows  into  the  starting  well  No.  1  and  the  starting 
bell  being  filled  with  air,  pressure  is  developed  which  is 
transm  tted,  as  shown  by  the  arrows,  to  the  blow-off 
trap  connected  with  siphon  No.  2.  When  the  discharge 
line  is  reached,  sufficient  head  is  obtained  on  the  starting 
bell  to  force  the  seal  in  blow-off  trap  No.  2,  thus  releasing 
the  air  confined  in  siphon  No.  2  and  bringing  it  into  full 
operation. 


510  AUTOMATIC   DOSING  DEVICES 

During  the  time  that  siphon  No.  2  ,.  is  operating, 
siphonic  action  is  developed  in  the  draining  siphon  con- 
nected with  starting  well  No.  2  and  as  soon  as  the  level 
in  the  tank  is  below  the  top  of  the  well  it  is  drained  down 
to  a  point  below  the  bottom  of  starting  well  No.  2.  It 
can  now  be  seen  that  after  the  first  discharge  starting  well 
No.  2  is  empty,  whereas  the  other  three  are  full.  .  .  .  There- 
fore when  the  tank  is  filled  the  second  time,  pressure  is 
developed  in  starting  bell  No.  2,  which  forces  the  seal  of 
blow-off  trap  No  3,  thus  starting  siphon  No.  3.  ... 

This  alternation  can  be  continued  for  any  number  of  siphons. 
Other  arrangements  have  been  devised  for  the  automatic  con- 
trol of  alternating  siphons,  but  these  principles  of  the  air-locked 
devices  are  fundamental. 

287.  Timed    Siphons. — In   the   operation   of   a   number   of 
contact  beds  not  only  must  the  dosing  of  the  tanks  be  alternated , 
but  some  method  is  needed  by  which  the  beds  shall  be  automat- 
ically emptied  after  the  proper  period  of  standing  full.     To  fulfill 
this  need  the  principle  of  the  timed  siphon  must  be  employed 
in  conjunction  with  the  alternating  siphons.     Fig.  185  illustrates 
the  operation  of  the  Miller  timed  siphon.     Its  operation  is  as 
follows:     water  is  admitted  to  the  contact  bed  and  transmitted 
to  the  main  siphon  chamber  through  the  "  opening  into  bed." 
Water  flows  from  the  main  siphon   chamber  into  the  timing 
chamber  at  a  rate  determined  by  the  timing  valve.     The  con- 
tact bed  is  held  full  during  this  period.     As  the  timing  chamber 
fills  with  water  air  is  caught  in  the  starting  bell  and  the  pressure 
is  increased  until  the  seal  in  the  main  blow-off  trap  is  blown  and 
the  main  siphon  is  put  into  operation.     As  the  water  level  in  the 
main  siphon  chamber  descends,   water  flows  from  the  timing 
chamber   into   the   main   siphon   through   the   draining   siphon 
and  the  tuning  chamber  is  emptied,  ready  to  commence  another 
cycle. 

288.  Multiple  Alternating  and  Timed  Siphons.1 — The  alter- 
nating and  timing  of  a  number  of  beds  is  more  complicated. 
The  arrangement  necessary  for  this  is  shown  in  Fig.   186.     It 
will  be  assumed  at  the  start  that  all  beds  are  empty  and  that  all 
feeds  are  air  locked  as  shown  in  Section  A  B  except  that  to  bed 
No.  4  into  which  sewage  is  running.     As  bed  No.  4  fills,  sewage 

1  Design  of  Siphon  by  G.  H.  Bayles,  Eng.  News-Record,  Vol.  84,  1920, 
p.  974. 


MULTIPLE  ALTERNATING  AND  TIMED  SIPHONS       511 


is  transmitted  through  the  opening  in  the  wall  into  the  timed 
siphon  chamber  No.  4.  When  the  level  of  'the  water  in  the  bed 
and  therefore  in  this  chamber  has  reached  the  top  of  the  with- 
draw siphon  leading  to  the  compression  dome  chamber  No.  4, 
this  latter  chamber  is  quickly  filled.  The  air  pressure  in  starting 
bell  No.  4a  is  transmitted  to  blow-off  trap  No.  la.  The  seal 
of  this  trap  is  blown,  releasing  the  air  lock  in  feed  No.  1  and  the 


Contact 
Bed 


Air  Vent  Is  not  necessary 
where  Siphon  discharges 
info  an  open  Carrier,  or 
Outlet  is  not  more  than 
50  feet  away. 


FIG.  185.— Miller  Timed  Siphon. 

Courtesy,  Pacific  Flush  Tank  Co. 

flow  into  bed  No.  1  is  commenced.  At  the  same  time  the  air 
pressure  in  compression  dome  No.  4  is  transmitted  to  feed  No.  4, 
air  locking  this  feed  and  stopping  the  flow  into  bed  No.  4.  The 
alternation  of  the  feed  into  the  different  beds  is  continued  in  this 
manner. 

Bed  No.  4  is  now  standing  full  and  No.  1  is  filling.  When 
compression  dome  chamber  No.  4  was  filled,  water  started 
flowing  through  timing  siphon  valve  No.  4  into  timing  chamber 


512 


AUTOMATIC  DOSING  DEVICES 


No.  4  at  a  rate  determined  by  the  amount  of  the  opening  of  the 
timing  valve.  As  this  chamber  fills  compression  is  transmitted 
to  blow-off  trap  46  and  when  sufficiently  great  this  trap  is  blown 
and  timed  siphon  No.  4  is  put  into  operation.  Bed  No.  4  is 
emptied  by  it,  and  compression  dome  chamber  No.  4  is  emptied 


C.LMei 


Marring  Bell  No.4  a. 


-Inlet 


Blow-off  Trap  No.  la 
Bed   No.  I 


"Blow-off  Trap 


FIG.  186.— Plural  Timed  and  Alternating  Siphons  for  Contact  Bed  Control. 

Courtesy,  Pacific  Flush  Tank  Co. 

through  the  withdraw  siphon  at  the  same  time.  This  com- 
pletes a  cycle  for  the  filling  and  emptying  of  one  bed  and  the 
method  of  passing  the  dose  on  to  another  bed  has  been  explained. 
The  principle  can  be  extended  to  the  operation  of  any  number 
of  beds. 


INDEX 


A.  B.  C.  process  of  sewage    treat- 
ment, 4 

Abandonment  of  contract,  225 
Access  to  work,  228,  229 
Accident,  contractor's  responsibility, 

221,  224 

Acetylene,  explosive,  347 
Acid  precipitation.     See  Miles  Acid 
Process. 

of  sludge,  503 

Acids  as  disinfectants,  489,  490. 
Activated  sludge.     Chapter  XVIII, 
465^79 

advantages     and     disadvantages, 
469,  470 

aeration  tank,  471,  472 

air  diffusion,  475,  477 

air  distribution,  473-478 

air  quantity,  475,  476 

area  of  filtros  plates,  478 

colloid  removal,  358 

composition,  465-469 

cost,  478,  479 

definition,  466 

dewatering,  468,  469,  497-505 

fertilizing  value,  469,  470 

historical,  470,  471 

how  obtained,  478 

nitrogen  content,  468 

patent,  471 

process,  465 

quantity,  469 

reaeration  tank,  473 

results,  467,  468,  476 

sedimentation  tank,  472 
Advertisement,  214 


Aeration,  effect  on  oxygen  dissolved, 
373-375 

of  sewage,  371,  376,  465-479 
Aerobes,  363 

Aerobic  decomposition,  366,  367 
Aftergrowths,  492 
Aggregates,  specifications,  172-174 
Air,   see  also  ventilation,   activated 
sludge,  compressed  air,  etc. 

ejectors,  150 

lock  dosing  apparatus.  Chap.  XXI, 
506-512 

machinery   for  activated   sludge, 

473,  474 
Algae,  363 
Alkalinity,  358 
Alleys,  sewers  in,  80 
Alum,  407,  408 
Alvord  tank,  427,  429 
Ammonia,  366,  367,  374,  375,  410 

explosives,  297 
Analyses,  bacteriological,  364 

chemical,  354,  355 

mechanical  of  sand,  182 

physical,  352-354 

sewage,  352-364 
Anerobes,  363,  365-367 
Anaerobic,  action,  410 

bacteria,  363 

conditions,  367 

decomposition,  365-367 
Ann  Arbor,   Michigan.     Population, 

14 
Annual  expense,  method  of  financing, 

157,  158 
Ansonia  air  ejector,  150,  151 


513 


514 


INDEX 


Antibiosis,  definition,  363 
Appurtenances  to  sewers.     Chap.  VI, 

99-115 
Arch,  analyses,  204-208 

elastic  method,  206-208 
vouissoir  analysis,  204-206 
brick  construction,  312,  313 
centers  for  brick  sewers,  313 
concrete  construction,  318-321 
Ardern  and  Lockett,  development  of 

activated  sludge,  467,  468,  471 
Area  of  cities,  31 
Asphyxiation  in  sewer  gas,  336 
Assessments,  special,  15,  16 
Augers,  earth,  21 
Automatic,  regulators,  117-121 
siphons,  flush  tanks,  110 
double  alternating,  507 
multiple  alternating,  508-512 
timed,  510 
timed  and  multiple  alternating, 

510-512 
triple  alternating,  508 

Bacillus,  definition  and  morphology, 

362,  363 

Backfilling,  328-331 
Backfill,  puddling,  330 
weight  of,  199,  201 
Backwater  curve,  73 
Bacteria,  definition  and  morphology, 

362,  363 

good  and  bad,  363,  364 
nature  of,  362,  363 
nitrifying,  431,  432 
sanitary  significance  of,  364 
in  sewage,  362,  363 
total  count,  364 
Bacterial  analyses,  results  in  sewage, 

364 

Baffles,  scum,  404,  413,  414,  421 
in  sedimentation  tanks,  404 
in  septic  tanks,  413,  414 
in  Imhoff  tanks,  421 
Balls,  for  cleaning  sewers,  338 
Band  screen,  384 
Barring,  definition,  263 
Bars  for  screens,  390 


Basins,  sedimentation,  baffling,  404 

bottoms,  404 

cleaning  arrangements,  404 

depth,  401 

economical  dimensions,  401-403 

inlets  and  outlets,  404 

scum  boards,  404 

types,  395 
Basket    handle    sewer    section,    67, 

69 

Bathing  beaches,  pollution,  381 
Bazin's  formula,  54 
Bearings,  for  centrifugal  pumps,  131, 
137,  138 

thrust,  138 
Bellmouth,  121,  122 
Bends  in  pipe,  loss  of  head  in,  116 
Berlin,  sewage  farm,  460,  461 

sewers,  date  of,  3 
Bids,  proposal,  217-219 
Bidder's  duties,  215-217 
Bio-chemical  oxygen  demand,  359— 

361 

Biolysis  of  sewage,  366,  367 
Black  and  Phelps  dilution  formulas, 

377-379 
Blasting  and  explosives,  294-304 

caps,  297,  299,  300 

detonators,  294,  297-300 

firing,  302-304 

fuses  and  detonators,  297-300 

fuses,  delayed  action,  291,  300 

fuses,  electric,  299,  300 
splicing,  303 

gelatine,  296 

loading  holes,  303 

powder,  295 

precautions,  300-302 

priming  and  loading,  303 

rock,  269 

size  of  charge,  304,  305 

tunneling,  290,  291 
Bleach,    characteristics    of    for    dis- 
infection, 491 
Block  sewer,  construction,  311-314 

hollow  tile  as  underdrains,  126 
Blocks,  vitrified  clay,  189,  190 
Boilers,  steam,  147-150 


INDEX 


513 


Boilers,  efficiencies,  149 

horse-power,  149 
Bond,  contractor's,  213,  214,  232 

issues,  14 

Bonds,  definition  and  types,  14-16 
Boring  underground,  20 
Bottom,    activated    sludge    aeration 

tank,  472 
Imhoff  tanks,  423 
sedimentation  tanks,  404 
trickling    filter,    451,  452 
Box  sheeting,  272 
Branch  sewer,  defined,  7 
Breast  boards,  288 
Brick,  arch  construction,  312,  313 
and  block  sewer  construction,  311- 

315 

invert  construction,  311,  312 
sewer  construction,  311-315 
arch  centers,  313 
invert,  311-312 
organization,  314,  315 
progress,  314 
row  lock  bond,  312 
specifications,  188,  189 
sewers,  life  of,  351 
Bricks  for  sewers,  316 
British  Royal  Commission  on  Sewage 

Disposal,  4 

Broad  irrigation.     See  under  Irriga- 
tion. 

Bucket  excavators,  246,  255,  256 
Building  material,  weight  of,  201 
Burkli-Ziegler  formula,  47,  425 
Butryn,  366 

Cableway  excavators,  246,  250-252 
Cage  screen,  384,  385 
Caisson  excavation,  285,  286 
Calcium  carbide,  explosive,  347 
Calumet  pumping  station,  128,  142 
Cameron  septic  patent,  411 
Capacity  of  sewers,  diagrams,  57-60 
Capital,  private  invested  in  sewers,  17 
Capitalization,  method  of  financing, 

157-160 

Caps,  blasting.    See  blasting. 
Carbohydrate,  366,  367 


Carbon,  analysis  for,  356 

dioxide,  366,  367 
Carson  Trench  machine,  250,  251 
Cast-iron  pipe,  122,  164,  190,  191 

joints,  164 

quality,  101,  102,  190 
Castings,  iron,  101,  102 
Catch-basins,  99,  107-108,  217 

cleaning,  343,  344 

inspection,  337 
Catenary  sewer  section,  69 
Cellars,  depth  of,  88 
Cellulose,  367 
Cement.     See  also  Concrete. 

pipe,    specifications,    manufacture 
and  sizes,  171-179 

vs.  concrete,  164 
Centrifugal     pumps.       See     pumps, 

centrifugal. 

Centrifuge  for  sludge  drying,  501,  502 
Cesspool,  411,  416,  417 
Champaign,  Illinois,  septic  tank,  415, 

416 

Changes  in  plan,  222,  223 
Channeling,  definition,  263 
Character  of  surface,  44 
Chemical  analyses,  354-362 
Chemical  precipitation,  371,  405-409 

chemicals  used,  405-407 

preparation  of  chemicals,  407, 
408 

results,  408,  409 

at  Worcester,  408 
Chezy  formula,  52,  53 
Chicago.    See  also  Sanitary  District 
of  Chicago. 

drainage  canal,  374,  375 

dilution   requirement   for  sewage, 
380 

early  sewers,  3 

method  of  sewage  disposal,  374 

population  and  density,  29,  30 

trench  excavation  in,  248 
Chlorine.    See  also  Disinfection. 

disinfectant,  489-493 

in  sewage,  358,  374,  375 
Chlorine  liquid,  application,  491,  492 
Cholera,  transmittable  disease,  364 


516 


INDEX 


Chromatin,  365 
Chutes  for  concrete,  187 
Circular  sewer  section,  hydraulic  ele- 
ments, 65,  66,  69 

types,  70,  71 
City,  growth  of  area,  31 

growth  of  population,  24-28 

legal  powers,  219 
Clay,  life  of  pipe,  349-351 

manufacture  of  pipe,  165-167 

specifications  for  pipe,  168-170 

unglazed  for  pipe,  165 

vitrified  blocks,  167,  189,  190 

vitrified  pipe,  165-171 
Cleaning,  grit  chambers,  398,  400 

sedimentation  basins,  404 

sewers,  cost,  341 

in  N.  Y.  City,  332 

methods,  337-343 

tools,  338-340 

up  after  completion  of  work,  228 
Coccus,  362 

Coefficient  of  uniformity  of  sand,  456 
Coffin  sewer  regulator,  117,  118 
Colloid,  nature  of,  358 

treatment  for,  358 
Color  of  sewage,  352,  353 
Combined  sewer  system,  78,  79 
Commercial  districts,  characteristics 

of  and  sewage  from,  32,  34,  35 
Compensators  for  pumps,  142 
Compressed  air.    See  also  ventilation, 
tunneling,  drilling,  etc. 

activated  sludge,  473-475 

for  drilling,  264-268 

in  tunnels,  292-294 

transporting  concrete,  320,  321 
Concentration,    time   of   flood   flow, 

41-43,  96,  97 
Concrete,  aggregates,  172-174 

mixing  and  placing,  184-188 

pipe,  details,  175-179 
manufacture,  171-179 
reinforcement,     177,    178,    209, 
210 

pipe,  steam  process,  176 
sizes,  175 

pressure  against  forms,  232,  323 


Concrete,  proportioning,  179-183 
qualities,  179,  180 
reinforcement,    placing,    178,  326, 

327 

reinforcing  steel,  quality,  191 
sewer  construction,  314-328 

arch,  318-321 

form  length,  319 

labor  costs,  327,  328 

in  open  cut,  314-320 

in  tunnel,  320,  321 

invert,  315-320 

organization  for,  328 

working  joints,  319 
sewer  costs,  327-329 
strength,  181 
waterproofing,  184 
Conduits,  special  sections,  67,  70,  71 
Connections   to   sewers,    ordinances, 

344,  345 

record  of  92,  238 
Construction  of  sewers,   Chap.  XI, 

233-331 
Construction,  elements  of,  233 

organizations,  315,  328 
Contact  bed,  432-437,  506 
advantages  and  disadvantages, 

432-434 

automatic  control,  437,  506 
cleaning,  435 
clogging,  435 
construction,  434-436 
control,  437,  506 
cycle,  436,  437 
depth,  434 
description,  432,  433 
design,  434-436 
dimensions,  434,  435 
loss  of  capacity,  435 
material,  435,  436 
multiple,  433,  435 
operating  conditions,  432-437 
rate,  435 
results,  433,  434 
ripening,  432 
Continuous  bucket  excavators,  246- 

250 
Contour  interval  on  maps,  79,  80 


INDEX 


517 


Contracts,  Chap.  X,  211-232 

abandonment  of,  225 

assignment,  228 

completion  of,  222,  228 

bond,  213,  222 

content,  213,  230,  231 

cost-plus,  212,  213 

disputes,  220 

divisions  of,  213 

drawings,  213 

engineer  as  an  arbitrator,  220 

the  instrument,  230,  231 

interpretation      of,      220,      234, 
235 

lump  sum,  212 

nature  of,  211,  212 

sample,  230,  231 

time  allowed,  222 

types,  212,  213 

unit  price,  213 
Contractor,  absence  of,  222 

bond,  232 

claims  against,  228 

duties,  221 

liability,  224 

relations   with    other    contractors, 

228,  229 

Contractor's  powder,  294 
Control  devices,  automatic,  for  sew- 
ers, 117-121 
for  filters,  506-512 

inspection  of,  336,  337 
Copper  sulphate,  disinfectant,  490 
Copperas,  precipitant,  406-408 
Cordeau  Bickford,  298,  303 
Corrugated  iron  pipe,  165 
Cost.    See  under  item  wanted. 
Cost,  annual.     Method  of  financing, 
157-160 

capitalized.    Method  of  financing, 
157-160 

classification  of,  235-238 

comparisons     of.       Methods     for 
making,  157-160 

collection  of  data,  10-14,  235-238 

estimate.       Method    of    making, 
10-14 

overhead,  237,  238 


Couplings,  flexible  for  shafts,  138 
Covers,  Imhoff  tanks,  424 

septic  tanks,  415 

trickling  filters,  451 
Crops  on  sewage  farms,  463,464 
Cunette,  67,  70 

Cut,  depth  of  excavation,  88,  92. 
Cycle,  contact  bed,  436 

life  and  death,  367,  431 

nitrogen,  367,  368 

trickling  filter,  441 
Cylinders,  stresses  in,  194,  202-204 
Cytoplasm,  365 

Damages,  liquidated,  222 

material,  221,  224 
Darcy's  formula,  52 
Day  labor,  211 

Decomposition  of  sewage,  365-367 
Definitions.     See  word  defined. 
Deflagration,  definition,  294 
Delays  in  contract  work,  228 
Delayed  action  fuses,  291,  300 
Densities.     See  population. 
Depreciation,  of  sewers,  348-351 

rate  of,  financial,  158 
Depth  of  sewers,  88 
Design  conditions,  88-92 

economical,  mathematics  of,  401- 
403 

preparations  for,  17-23 
Detention     period,     grit     chamber, 
397 

Imhoff  tank,  419 

plain  sedimentation,  392-395,  401 

septic  tank,  415 
Detonation,  definition,  294 
Detonator.     See  blasting  cap. 
Diameter  of  sewers,  57-60,  72,  88- 

92 

Diaphragm  pump,  257,  258 
Diesel  engine,  152,  154 
Digestion  chamber,  Imhoff  tank,  422, 

423 
Digestion  of  sludge  in  separate  tank, 

427-430,  497 
Dilution,  amount  needed,  377-380 

conditions  for  success,  372,  373 


518 


INDEX 


Dilution,  definition,  372 

formulas  for  quantity,  378-380 

governmental  control,  380,  381 

preliminary  studies,  381,  382 

in  salt  water,  376,  377 

in  streams,  372-376 

of   sewage,  370   and  Chap.  XIV, 

372-382 

Diseases,  water-borne,  364 
Disinfection,  489-493 

action  of,  489-491 

bleaching  powder,  491 

chlorine,  liquid,  491 
amount  of,  492 

disinfectants,  489,  490 

purpose,  489 

selective   action    of    disinfectants, 

492,  493 
Disk  screen,  384 

Disposal  of  sewage,   See  sewage  treat- 
ment. 

Disputes,  engineer  to  settle,  220 
Dissolved  oxygen.     See  Oxygen  dis- 
solved. 
Distribution  of  sewage, 

contact  beds,  436 

irrigation,  461,  462 

nozzles,  442-449 

sand  filter,  450-458 

traveling  distributor,  442 

trickling  filters,  441-451 
Districts,  character  of,  29,  30,  32-37 

classification  of,  34,  35 
Domestic  sewage,  defined,  6,  7,  352 
Dorr  Thickeners,  472,  504 
Dortmund  tank,  404 
Dosing  devices,  506-512 

alternating    and     timed    siphons, 
500-512 

Alvord  device  at  Lake  Forest,  506 

four  or  more  alternating  siphons, 
509 

operation    of    automatic    siphon, 
110 

three  alternating  siphons,  508 

timed  siphons,  510 

two  alternating  siphons,  507 
types,  506 


Dosing    tank    design,    for    trickling 

filter,  446-450 
Doten  tank,  429,  430 
Drag  line  excavators,  255,  256 
Drainage  areas,  81,  84,  94 
Drills,  electric,  267 

jack  hammer,  264,  265 

punch,  20 

size  of  cylinder  for,  266 

tripod,  264,  265 
Drilling,  methods,  20-23,  264-270 

depth,   diameter   and    spacing  of 
holes,  268-270 

power  for,  267,  268 

rate  of,  in  rock,  267 

steam  and  air,  267,  268 
Drop  manhole,  100,  101 
Drop-down  curve,  73,  77 
Drum  screen,  384 
Dry-weather  flow,  24,  38 
Drying  sludge.    See  sludge  drying. 
Dualin,  296 
Duty  of  contractor.    See  Contractor, 

duties 
Duty    of    engineer.      See    Engineer, 

duties. 
Duty  of  inspector.     See  Inspector, 

duties. 

Duty  of  a  pump,  defined,  135 
Dynamite,    296-298,    300-302,    304, 
305 

cartridge,  268,  296,  302 

thawing,  301,  302 
Dysentery,  365 

Earth  pressures,  theories,  274,  275 
Economical  dimensions,  mathematics 

of,  401-403 

Effective  size  of  sand,  defined,  456 
Efficiency  of  a  pump,  defined,  135 
Effluents,  character  of 

activated  sludge,  467,  468 

chemical  precipitation,  408 

contact  bed,  434 

Imhoff  tank,  414,  424,  425,  432 

lime  and  electricity,  489 

Miles  acid  process,  484,  485 

sand  filter,  453 


INDEX 


519 


Effluents,  sedimentation  tank,  401 

septic  tank,  412-414 
Egg-shaped  section,  67,  68,  70 
Ejectors,  air,  150,  151 
Elastic  arch  analysis,  206-208 
Electric  motors,  150-152 
Electrolytic  treatment,  487-489 
Elevations,  method  of  recording,  92 
Emergencies,  duties  of  engineer,  235 
Emerson  pump,  261 
Engines,   internal  combustion,    152- 

154 

steam,  types,  142-144. 
Engineer,  absence  of,  221 
defined,  220 

disputes  settled  by,  220,  234 
duties    of,    9,    10,   220,  233,  234, 

238 
individuality  and   personality,    9, 

234 

qualifications,  9 
sanitary,  definition,  2 
Engineering  News  pile  formula,  125, 

126 
Entering    sewers,    precautions,    335, 

336 

Enzymes,  365 

Equipment  for  construction,  237 
Equivalent  sections,  defined,  72 
solution  of  problems  in,  67-72 
Estimates,  cost  and  work  done,  10-14 
when  made,  226 
data  for,  235 

Excavation,  depth  of  open  cut,  284 
drainage,  252,  262 
hand,  242-245,  249 
economy,  245 
laborer's  ability,  243 
lay  out  of  tasks,  243 
Excavation,    hand,   opening  trench, 

243 

vs.  machine,  245,  249 
tools,  242 
machine,  244-246 
economy,  245 
limitations,  246 
vs.  hand,  245,  249 
specifications,  240,  241 


Excavating  machines,  bucket,  246, 255 

cableway   and   trestle,    246,    250- 
252 

Carson  machine,  250,  251 

continuous  belt,  246 
bucket,  246,  247 

drag  line,  255 

Potter  machine,  251 

steam  shovel,  252-254 

tower  cableway,  252 

wheel  excavators,  246-250 
Excavation,    machine,    organization, 
249 

pumping  and  drainage,  256,  257 

quicksand,  256 

rock,  263  264 
payment  for,  230 

specifications,  240,  241 

trench  bottom,  241,  304,  311 
Explosions  in  sewers,  108,  336,  346- 
348 

causes  of,  346 

historical,  346 

prevention,  108,  348 
Explosives.    See  also  Blasting. 
Explosives,  and  blasting,  294-304 

ammonia  compounds,  297 

blasting  gelatine,  296 

contractor's  powder,  294 

deflagrating,  294 

detonating,  294 

detonators,  294,  297-300 

"  Don'ts,"  300,  301 

dynamite,  296-298,  300-302,  304, 
305 

fuses  and  detonators,  297-300 

gelatine  dynamite,  296 

gunpowder,  295 

handling,  300-302 

nitro-glycerine,  295 

nitro  -  substitution    compounds, 
295 

permissible,  297 

quantity,  304,  305 

requirements,  294 

strength  of,  297,  298 

T.N.T.,  295 

types,  294-297 


520 


INDEX 


Exponential    formulas    for    flow    of 

water,  54,  55 
Extra  work,  compensation,  227 

Facultative  bacteria,  363 
Farming's  run-off  formula,  49 
Farms,  septic  tanks  for,  416,  417 
Farming  with  sewage.    See  irrigation. 
Fats  in  sewage,  357-359,  366,  367 

from  Miles  acid  process,  485-487 
Feathers,  for  splitting  rock,  264 
Ferrous  sulphate,   precipitant,   406- 

408 

Fertilizer  from  sludge,  470,  495,  497 
Fertilizing  value  of,  activated  sludge, 
470 

sewage,  459,  460 
Filter  press  for  sludge,  500,  501 
Filters.    See  under  name  of  filter. 
Filtration,  of  sewage,  370, 371, 431-459 

action  in,  theory  of,  431 

cost,  458,  459 
Filtros  plates,  477,  478 
Finances,  mathematics  of,  157-160 
Financing,  methods  of,  14-17 
Flamant's  formula,  54,  56 
Flies  on  trickling  filters,  438 
Flight  sewer,  101,  102 
Flood,  crest  velocities,  42,  43 

flow  computations,  94-98 
McMath  formula,  94,  96,  97 
Rational  method,  95-98 
Flow,  laws  of,  52 

velocity  of,  52,  90,  91 
Fluctuations,  in  rate  of  sewage  flow, 
33-38 

in  quality  of  sewage,  368-370 
Flush  tanks,  automatic,  109-113 

capacity,  111 

details,  110,  112 

inspection  of,  336,  337 

payment  for,  217 

siphon  sizes,  111 
Flushing,  109-113,  341-343 

amount  of  water  needed,  112 

methods,  341-343 

manhole,  109 

sewer,  defined,  8 


Foaming  of  Imhoff  tanks,  425,  426 
Foot  valves,  141 
Force  main,  denned,  8 
Forms,  design  of,  322,  323 

length  of,  319 

materials,  321,  322 

oiling,  174,  186,  322 

specifications,  322 

steel,  325,  326 

steel  lined,  325 

support  for,  316,  318 

time  in  place,  319 

wooden,  323,  324 

Formulas,    hydraulic,     methods    for 
solution,  55-61 

for  flow  of  water,  52-55 

for  rainfall.    See  Rainfall. 

for  run-off.    See  Run-off. 
Foundations,  99,  124-126 
Franchises  for  sewers,  17 
Free  ammonia,  366,  367,   374,  375, 

410 
Freezing,  catch-basins,  108 

concrete,  186,  187 

dynamite,  301,  302 
Fresh   sewage,    characteristics,    352- 

354 
Friction  losses.    See  Head  losses. 

flow  in  pipe,  51,  52 
Fuel,  consumption  by  prime  movers, 
153 

costs,  153 

heat  value,  150 
Fungus  growth  in  sewers,  333 
Fuses.    See  blasting  fuses. 

Ganguillet  and  Kutter's  formula,  52- 

65 
Gas,  chamber  in  Imhoff  tank.     See 

Scum  chamber, 
engines,  152-154 
illuminating,  explosive,  347 
sewer,  335,  336 
Gasoline,  explosive,    108,    109,    335, 

346,  347 

engines,  152-154 
and  oil  separator,  109 
odors,  significance,  335,  353 


INDEX 


521 


Gearing,  reduction  for  turbines,  140, 

146 

Gelatine  dynamite,  296 
Glycerol,  366 
Gothic  section,  67 

Governmental   control,    stream  pol- 
lution, 380,  381 
Grade,  of  sewers.    See  also  Slope. 

how  given,  281-284 

selection  of,  90 
Stakes,  221,  281-283 
Gravel,  specifications,  172 
Grease,  in  sewers,  99,  108,  333,  345 
cutter,  340 

ordinance  concerning,  345 
traps,  99,  108 
Gregory's   imperviousness  formulas, 

44,  46 

Grit,  cloggs  sewers,  333 
chambers,  127,  397-401 

description,  395,  398 

design,  397,  398 

dimensions,  397,  398 

existing,  398-400 

outlet  arrangements,  400 

results,  397 

retention  period,  397 

sludge  analyses,  397 

units,  number  of,  400,  401 

velocity  of  flow  in,  396-398 
quantity  and  character  of,  397 
Gooves  in  concrete,  working   joints, 

319 
Ground  water  in  sewers,  38,  39,  85, 

87,  256,  352 
Gun  cotton,  296 
Gunpowder,  295 

Hazen,  theory  of  sedimentation,  392- 
395 

dilution  formula,  380 
Hazen  and  William's  formula,  55,  57 
Head  loss,  in  bends,  116 

entrance,  115 

friction  in  straight  pipe,  51,  52,  115 
Hercules  powder,  296 
Bering,    Rudolph,    dilution    recom- 
mendations, 380 


Hering,     Rudolph,     introduction  of 
Imhoff     tank     and     hydraulic 
formulas,   425 
Historical  resume  of  sewerage  and 

sewage  treatment,  2-5 
Hitch,  tunnel  frame,  286,  287 
Holes,  drill.    See  Drill  holes. 
Holidays,  work  on,  221 
Hook  for  lifting  pipe,  304,  306 
Horse-power,  boiler,  149,  150 

of  pumps,  144-146 
Horse-shoe  sewer  section,  71 
House,  connections,  record  of,  92,  234 

drains,  7,  88,  90 

sewer,  defined,  7 
Hydraulic,  elements,  65,  69 

formulas,  52-55 

jump,  73-74 

principles,  51,  52,  72,  73 

value  of  settling  particles,  393 
Hydraulics  of ,  sewers,  Chap.  IV,  51- 
77 

circular  pipes  partly  full,  65,  66 

equivalent  sections,  72 

non-uniform  flow,  72-77 

sections  other  than  circular,  67-72 

use  of  diagrams,  61-65 
Hydrocarbon,  367 
Hydrogen  sulphide,  353,  366,  410 
Hydrolytic  tank,  427,  428 
"  Hypo  "  as  a  disinfectant,  491 
Hyto  Turbo  blower,  473,  474 

Illinois  River,  self-purification,  374- 

376 

Imhoff  tank,  and  chlorination,  costs, 
487 

cover,  424 

description,  417-419 

design,  419^24 

digestion  chamber,  422 

inlet  and  outlet,  421 

operation,  426-427 

patent,  418 

results,  414,  424,  425,  439,  467 

sedimentation  chamber,   419-422 

scum  chamber,  424 

slot,  422 


522 


INDEX 


Imhoff  tank,  sludge,  414,  467 
sludge  pipe,  423,  424 
status,  425,  426 
and  trickling  filter,  cost,  479 
Impeller,  for  centrifugal  pump,  131, 136 
Imperviousness,  relative,  40,  42,  44- 

46,  95-97 

Industrial,  districts,  32-37 
wastes,  denned,  7,  352 

tannery,  491 
Information     and     instructions     for 

bidders,  213,  215-217 
Inlets,  street,  93,  94,  99,  104-107 
Inspection,   contract  stipulations, 

221-224 

during  construction,  233,  234 
for  maintenance,    104,    333-337, 

348,  349 

Inspector,  absence  of,  221,  222 
duties,  233-234 
qualifications,  234 
Institutional  sewage  treatment  plants, 

416,  417 

Intercepting  sewer,  defined,  7 
Intermittents  and  filter.      See  Sand 

filter. 

Internal  combustion  engines,  152-154 
Inverted  siphon,  113-116 
Iron,    ferrous   sulphate,    precipitant, 

406-408 

cast.     See  cast  iron. 
Irrigation.     See   also   Farming   and 

Sewage  farming, 
area  required,  463 
Berlin  sewage  farm,  460,  461 
crops,  463,  464 
description,  459 
fertilizing  value  of  sewage,  460,  470, 

495,  498 
vs.  farming,  459 
operation,  461-463 
preliminary  treatment,  462,  463 
preparation  for,  461-463 
process,  459,  460 
sanitary  aspects  463 
status,  460,  461, 
theory,  432 
in  the  United  States,  461 


Jack  hammer  drill,  264,  265 
Jetting  method,  21-23 
Jet  pump,  259,  341,  343 
Joints,  bituminous,  309-311 

in  cast-iron  pipe,  164 

cement,  307,  308 

inspection  of,  234 

lead,  164 

mortar,  307 

open,  307 

poured,  309-311 
cement,  309,  311 

riveted  steel,  195,  196 

sulphur  and  sand,  309 

types,  for  pipe,  307 

working,  in  concrete,  319 
Junctions,  99 

Kuichling,  run-off  rules,  46,  47,  49 

storm  intensity  formulas,  50 
Kutter's  formula,  52-65 

Labor,  day  vs.  contract,  211 

costs  on  concrete  sewer,  328,  329 
Labyrinth  packing  rings,  136,  137 
Lagging,  tunnel  frames,  287 

for  forms,  322 
Lagooning  sludge,  495-497 
Laitance,  186,  188 
Lakes,  self-purification  of,  376 
Lampe's  formula,  54 
Lampholes,  99,  104 
Lateral  sewer,  defined,  7 
Lawrence  Experiment  Station,  4 
Leaping  weir,  118-121,  337 
Legal  requirements,  construction,  224 

dilution,  380,  381 

in  design,  9 
Liernur  system,  5 
Life,  organic  in  sewage,  363,  364 

of  sewers,  348-351 
Lime  as  a  precipitant,  405-408 

with  electricity,  488,  489 

with  iron,  406,  407 
Line  and  grade,  281-284 

how  given,  281-283 
Liquefaction  of  sludge,  411-413,  496, 
497 


INDEX 


523 


Liquid  chlorine.     See  also  Chlorine, 

491 

Liquidated  damages,  222 
Loads  on,  pipe,  198-202 

Mansion's  method,  198-202 
trench,  199-202 
Lock  bar  pipe,  197 
Lock  joint  pipe,  177 
Long  loads,  201 

Machine  excavation.  See  Excavation. 
Macroscopic  organisms,  363,  368 
Main  sewer,  defined,  7 
Maintenance  of  sewers,  Chap.  XII, 
332-351 

catch-basin  cleaning,  343,  344 

cleaning  sewers,  337-343 

complaints,  333 

cost,  341 

entering  sewers,  335,  336 

flushing,  109-113,  341-343 

hand  cleaning,  341 

inspection,  333-337 

organization,  332 

protection  of  sewers,  344,  345 

repairs,  337 

tools,  338-341 

troubles,  333 

work  involved,  332 
Man,  shoveling  ability,  243 
Manholes,  81,  99-104 

bottom,  100 

cover,  102-103 

drop,  101 

flushing,  109,  342 

location  and  numbering,  81 

payment  methods,  217,  218 

steps,  100,  103,  104 
Manning's  formula,  55 
Map,  preliminary,  17,  79,  80,  82,  83 
Marsh -gas,  347,  366,  367,  410,  415 
Marston's  methods  for  external  loads 

on  buried  pipe,  198-202 
Materials,   for   sewers,  Chap.  VIII, 

164-193 

measurement  of,  236,  237 
record  of,  237 

unit  weights,  201,  202 


McMath's  formula,  47,  48,  94,  95 
Meem's  theory  of  earth  pressure,  274, 

275 

Mercaptan,  367 
Metabolism,  365 
Methane,  347,  366,  367,  410,  415 
Methylene  blue,  360 
Microscopic  organisms,  363,  364,  368 
Miles  acid  process,  costs,  487 

amount  of  acid,  483 

analyses  of  sludge,  485 

description,  482 

results,  483-487 

sludge,  485 

Mineral  matter  in  sewage,  357 
Mirror,  inspecting  device,  334 
Money  retained  by  city,  227 
Mosquitoes  in  catch-basins,  108 
Motors,  electric,  150-152 
Municipal,  bond,  14,  15 

corporations,  15 


n,  value  of  in  Kutter's  formula,  53 
New  York  City,  density  of  population, 
29,31 

siphons  under  subway,  114 

grease  and  gasoline  trap,  108,  109 

aeration  of  sewage,  377,  470 

cleaning  sewers,  332 

depreciation  of  sewers,  348-351 
Needle  beam,  286,  287 
Night,  soil,  5 

work,  221 
Nitrates,  355,  356 
Nitrites,  355,  356 
Nitrifying  organisms,  431,  432 
Nitrobacter,  431,  432 
Nitro  explosives,  295,  296 
Nitrogen,  cycle,  367,  368 

organic,  355,  356 
Nitro-glycerine,  295 
Nitrosomonas,  431,  432 
Nomograph,  55,  56 
Non-uniform  flow,  72-77 
Nozzles.    See  also  Trickling  filters, 

coefficients  of  discharge,  446 

types,  445 


524 


INDEX 


Numbering,  drainage  areas,  81,  94 

manholes,  81 
Nye  steam  pump,  260,  263 

Obstructions  to  construction,  235 

Odor  of  sewage,  353 

Oil  in  sewage,  108,  344-348 

Oiling  forms,  174,  186,  322 

Olein,  366 

Ordinances,  for  protection  of  sewers, 

344,  345 
Organisms     in     sewage,     363,     364, 

368 

Organic  matter,  composition,  366 
Organizations  for  construction,  315, 

317,  328 

Orders,  to  whom  given,  222 
Outfall  sewer,  denned,  8 
Outlets,  99,  122-124,  373 
Overflow  weir,  118-121 

inspection  of,  337 
Overhead,  costs,  division  of,  10,  237, 

238 

-track  excavators,  246,  250,  251 
Oxidation  in  streams,  373-376 
Oxygen,  absorption  of,  374-377 
consumed,  355,  356 
demand,  359-361 
computation  of,  360 
bio-chemical,  359-361 
Oxygen  dissolved 
exhaustion  of,  366 
in  dilution,  381 
solubility,  362 
supersaturation,  361 
concentration  for  successful  dilu- 
tion, 377-380 
formulas  for  concentration,  378- 

380 
significance    of    in    sewage,    359- 

362 
Oysters,  contamination  of,  372,  489 

Packing  rings,   labyrinth  type,    136, 

137 

Palmatin,  366 
Parasites,  365 
Paris  sewage  farm,  460 


Patents.      Protection    of    City    by 

contractor,  224,  225 
Pathogenic  bacteria,  364 
Pavement,  replacing,  329 
Payment,  final  on  contract,  228 
Payments,  methods  of  making,  217, 

218 

Periscope  inspecting  device,  334,  335 
Permissible  explosives,  297 
Phenolphthalein  indicator,  408 
Photographic  records,  238 
Piles  for  foundations,  123-126 
Pills  for  cleaning  sewers,  338 
Pipe,  bedding,  230,  304,  328 

cast-iron.      See    under    cast   iron 
pipe. 

design  of  ring,  Chap.  IX,  194-210 

external  loads  on,  198-202 

joints.    See  Joints. 

sewer  construction,  304-311 

laying,  line  and  grade,  282-284 
organization,  311 

method  of  laying,  304,  306,  307 

steel,  design,  195-197 

stresses    in,    external    forces,   194, 
202-204 

stresses  due  to    internal   pressure, 
194 

stresses  in  buried  pipe,  198-204 

stresses  in  circular  ring,  202-204 

wood  design,  197,  198 
Plankton,  defined,  363 

in  sewage,  368 

Plans,  changes  in  contract,  222,  223 
Plug  and  feathers  for  splitting  rock, 

264 
Pneumatic,  collection  system,  5 

concreting,  320,  321 
Poling  boards,  in  open  cut,  271,  272 

in  tunnel,  287 

Pollution,  legal  features,  380,  381 
Population,  density,  28-31 

predictions,  24-27 

served  by  sewers  in  the  U.  S.,  3 

sources  of  information,  27,  28 

and  quantity  of  sewage,  31,  32 
Potter  trench  machine,  251 
Powder.     See  Blasting. 


INDEX 


525 


Power  pump,  132,  133 

Precautions  in  entering  sewers,  335, 

336 

Precipitants,  chemical,  405-407 
Preliminary,  map,  17,  79,  80,  82,  83 

work,  9,  17-23 
Present  worth,  158,  160 
Pressing  sludge,  500,  501 
Priming  explosives,  302-304 
Private,  capital,  17 

sewers,  17 
Privy,  5 

Profile,  for  brick  sewers,  312 
sewer,  92 
surface,  88 
Progress,  rate  of,  222 

reports,  238 

Promotion  (inception  of  sewers),  9 
Proportioning    concrete.      See    Con- 
crete proportioning. 
Proposal  (contract),  213,  217-219 
Protection    of   sewers    (ordinances), 

344,  345 
Protein,  366 
Puddling,  backfill,  330 
Pulsometer  pump,  260,  261 
Pumping,  in  excavations,  256-263 
selection  of  machinery,  154-156 
equipment,  cost  comparison,  162 
station,  128,  142 
costs,  156-163 
equipment,  127,  128 
Pumps,  air  ejector,  150,  151 
capacity,  129,  160-163 
capacity  of  units,  160-163 
centrifugal,  details,  130,  131,  136- 

138 

automatic  control,  141,  142 
characteristics,  138-140 
efficiency,  140 
for  excavation,  262 
motors  for  driving,  150-152 
performance,  138-140 
protection  of,  by  screens,  386 
selection  of,  154-156 
setting,  140-142 
turbine,  130-132,  154 
types,  130,  131 


Pumps,  centrifugal,  volute,  130-132, 

154 

character  of  load,  129 
costs,  156,  157 

description  of  types,  130-134 
for  construction  work,  256-263 
diaphragm,  257,  258 
direct-acting,  133 
duty  of,  135,  136 
efficiencies,  135,  136 
ejector,   134,  150,    151,   259,   341, 

343 

jet,  259 
need  for,  127 
number  of  units,  160-163 
packing  of,  133,  134 
piston,  133 

speed,  133,  134 
plunger,  133 
power,  132,  133 

reciprocating,   130,   132-135,   154- 
156 

for  excavation,  262 
reliability,  127 
sizes,  135 
steam,  134,  135,  142-146 

consumption,  144,  145 

vacuum,  259,  262 
improvised  for  trench  work,  257 
turbine,  130-132,  154 
volute,  130-132,  154 
Putrescibility,  359,  360 

Quantity,  of  sewage,  24-50,  84-87 

variations,  33-38 

storm  water,  40-50,  94-98 
Quicksand,  definition,  256 

excavation  in,  256 

safeguards,  235 
Quiescent  water,  self-purification,  374 

Racks.    See  Screens. 
Rainfall,  17,  40,  41,  50,  96,  97 

data,  17 

rate,  96,  97 

Rangers,  270-274,  276-279 
Rankine's  theory  of  earth  pressure, 
275 


526 


INDEX 


Rapid  sand  filtration  of  sewage,  458 
Rational  method  of  run-off  determi- 
nation, 40,  95-98 
Reaeration  tank  in  activated  sludge, 

473 

Receiving  well,  capacity,  129,  130 
Reciprocating  pumps.     See  Pumps, 

reciprocating. 

Records,  character  of,  on  construc- 
tion, 238-240 

Rectangular  sewer  section,  67-69 
Regulators,  99,  117-121,  337 

inspection  of,  337 
Reinforced    concrete    sewer    design, 

209,  210 
Reinforcing  steel,  specifications,  191 

placing,  326,  327 
Reinsch-Wurl  screen,  384 
Relative  stability  numbers,  359 
Relief  sewer,  defined,  7 
Repairs  to  sewers,  337 
Report,  engineer's  preliminary,  10 
Reservoir,    collecting   capacity,    129, 

130 

Residences,  septic  tanks  for,  416,  417 
Residential  districts,  characteristics, 

32-37 

Residue  on  evaporation,  356,  357 
Rideal's  dilution  formula,  379 
Ring,  design.    Chap.  IX,  194-210 

stresses  in  circular,  202-204 
River  pollution,  legal  features,  380, 

381 

Rivers,  self-purification  of,  373-376 
Riveted  joints,  properties,  196 
Rock,  blasting,  268,  290,  291 
definition,  263*' 
drill,  data  on,  266,  267 
drilling.    See  also  Drilling, 
by  hand,  264 
by  power,  264-268 
rates,  267 
excavation.    See  also  Excavation. 

payment  for,  230 
measurement  of,  in  place,  235 
tunnels,  290,  291 
Rods,  sewer,  338 
Roman  ordinance  relative  to  sewers,  2 


Roofs.*  See  Covers. 

Root  cutters,  340 

Roots,  333,  340 

Row  lock  bond  for  bricks,  312 

Running  water,  self -purification,  373- 

376 
Run-off,  computations,  17,  40,  46-50, 

94-98 

Safeguards  during  construction,  221, 

241 

Salt  water,  dilution  in,  376,  377 
Sand,  effective  size,  456 
uniformity  coefficient,  456 
filters,  452^59 
action  in  431,  432,  452^54 
control,  458,  506-510 
description,  452 
dimensions,  456 
distribution  systems,  433,  456-  , 

458 

dosing,  454-456 
dosing  devices,  506-510 
materials,  456 
operation,  454,  455 
preliminary  treatment,  455 
rate,  455 
results,  452,  453 
size  of  sand  for,  456 
thickness,  456 
in  winter,  455 

Sanitary  District  of  Chicago, 
dilution  factor,  380 
specifications, 

for  manhole  covers,  101,  102 
tunnel  cover,  284 
tunnel  ventilation,  291 
Sanitary  engineering,  1,  2 
Sanitary  sewage,  defined,  7,  352 
Saph  and  Schoder's  formula,  54 
Saprophytes,  365 
Screed,  316 
Screens,  383-391 

chlorination     and     fine     screens, 

costs,  487 
coarse,  386,  391 
data  on  fine,  388,  389 
design  of,  389-391 


INDEX 


527 


Screens,  fine,  381,  382,  387-389 
fixed,  385,  390 
medium,  386 

movable,  385,  386,  389-391 
moving,  384-386 
openings,  386-389 
protection  to  pumps,  127,  141 
purpose,  383 
results,  386-389 
size  and  performance,  386-389 
sizes,  386-391 
types,  384-386 

sewage  treatment  by,  371,  381 
Screening,  vs.  sedimentation,  383 

purpose,  object,  383 
Screenings,  character  of,  386-389 
Scum,  boards  for,  septic  tanks,  413, 

414 

Imhoff  tanks,  421 
chamber  in  an  Imhoff  tank,  424 
definition,  495 
Sediment,  velocity  of  transportation, 

396,  397 

Sedimentation,  383-405 
definition,  383 
Hazen's  analysis,  392-395 
hydraulic  values,  393 
a  method  of  treatment,  370 
object,  383 
Peoria  Lakes,  376 
protection  of  siphons,  113,  114 
results  from   plain    sedimentation 

401 

theory  of,  391-395 
transportation  of  debris,  396 
velocity  of,  392,  393 
vs.  screening,  383 
velocities,  limiting,  396,  397 
Sedimentation,  basins,  arrangement, 

394 

baffling,  404 
cleaning,  404 
dimensions,  401-403 
inlet  and  outlet,  404 
operation,  411 
types,  395 

chamber,  Imhoff  tank,  419-422 
Self-purification  of  lakes,  376 


Self-purification  of  streams,  373-376 
Separate  sewer  systems,  78-80 
Septic  action,  353,  365-368,  371,  410, 
411,  496,  497 

results,  412,  413 

vs.  sedimentation,  411 
Septic  tank,  411 

baffling,  413,  414 

capacities  of  small  tanks,  417 

for  country  homes,  416,  417 

covers  for,  415 

definition,  411 

design,  413^17 

explosions  in,  415 

results,  412,  413 

seeding,  413 

sludge  storage,  414 

small,  416,  417 

units,  415 

Septic  sludge  analysis,  414 
Septicization.    Chap.  XVI,  410-430 

a  method  of  treatment,  371 

the  process,  410,  411 

results,  412,  413 
Settling  solids,  357 
Sewage  and  water  supply,  32 

aeration,  371,  376,  465-479 

alkalinity  of,  358 

analyses,  chemical,  355,  369,  467 
interpretation  of,  356-362 
physical,  352-354 

average,  352-355 

bacteria,  362-365 

biolysis  of,  366,  367 

changes    in,  rate  of  discharge  of, 

33-38 
characteristics,  368-370 

characteristics  of,  352-354 

chemical  constitutents,  354-356 

classification  of,  6,  7,  352 

collection,  5 

color,  352,  353 

components  and   properties,  352— 
356 

decomposition  of,  365-367 

definition,  6,  7,  352 

disposal.    See  also  Sewage  treat- 
ment. 


528 


INDEX 


Sewage,  disposal,  methods,  6,  370,  371 

purposes,  370,  371 
domestic,  7,  352 
farming.    See  Irrigation, 
fertilizing  value,  459,  460 
flow  fluctuations,  33-38 

ratio  of  maximum  to  average, 

36,  37,  85 
fresh,  352-354 
gas,  335,  336,  353 
industrial,  denned,  7,  352 
life  in,  363-365,  368 
odor,  353 
physical,  analyses,  352-354 

characteristics,  352-354 
quality  variations,  368-370 
quantity.    Chap.  Ill,  24-50,  and 
84,  87 

and  population,  31,  32 

of  sanitary.  24-40 

variations,  33-38 
sanitary,  defined,  7,  352 
septic,     353,    365-368,    371,    410, 

411,  496,  497 
stability,  359,  360 
stale,  353 

storm,  defined,  7,  352 
strong,  355 
temperature,  353 
turbidity,  353 
treatment  processes,  370,  371 

A.  B.  C.,  4 

activated  sludge,  Chap.  XVIII, 
465-479 

biological,  371 

chemical,  371 

contact  bed,  432-437,  506 

costs,  459 

dilution.    Chap.  XIV,  372-382 

disinfection,  489-493 

electrolytic,  487-489 

filtration,  431^59 

increase  of,  3 

irrigation,  431,  459-464 

mechanical,  471 

Miles  acid  process,  482-487 

purpose  of,  6,  370 

resume,  6,  370,  371 


Sewage,    treatment    processes,    sand 

filter,  452-458 
screening,  383-391 
s'edimentation,  391^109,  411 
septicization.  Chap.  XVI, 4 10430 
trickling  niters,  437-452 

weak,  355 

and  water  supplies,  31,  32 
Sewerage,  definition,  7 

demand  for,  2 

design,  78-98 

growth  of,  2-4 

historical,  2-4 
Sewers,  ancient,  2,  3 

capacity,  diagrams,  56-60 

cost,  10-14 

definitions  of  various  types,  7,  8 

depth  of,  88 

diameter,  58-60,  88-92 

flat  grades,  73,  109 

flight,  101,  102 

inspection  of,  333-337 

life  of,  348-351 

location  of,  80,  81,  94 

materials.    Chap.  VIII,  164-193 

medieval,  3 

pipe,  properties  of  concrete,  175 
design.    Chap.  IX,  194-210 
vitrified  clay,  properties,  169-171 

profile,  89,  92 

section  of  different  types,  67-72 

separate  system,  78,  79,  82,  86,  87 

slope,  88-92 

storm-water    system,    78,    79,  83, 
93,94 

stresses  in,  194,  198-204 
Shafts,  for  tunnels,  284-287 
Sheeting,  270-280 

alignment,  240,  241 

backfilling,  330 

box,  272 

design,  275-280 

driving,  273 

length,  273 

lumber,  277 

moving,  248 

poling  boards,  271,  272,  287 

pulling,  274 


INDEX 


529 


Sheeting,  skeleton,  270,  271 
stay  bracing,  270 
steel,  252,  280,  281 
thickness,  276-278 
types,  270 

vertical,  270,  272-274 
Wakefield  piling,  273 
Shellfish  contamination,  372,  489 
Shields,  tunnel,  288-290 
Short  loads  on  trenches,  202 
Shovels,  for  hand  excavation,  242 

steam.    See  Steam  shovels. 
Shovel  vane  screen,  384 
Shoveling  by  hand,  height  raised,  244 

performance  by  one  man,  243 
Simbiosis,  definition,  363 

example,  432 
Sinking  fund,  158 

Siphons,  automatic.  Chap.  XXI, 
506-512.  See  also  under  Dosing 
devices. 

in  flush-tanks,  109-110 
inspection,  337 
operation,  109-110,  506-512 
for  trickling  filter,  448-451 
true  and  inverted,  113-117 
Skeleton  sheeting,  270,  271 
Slope,  of  sewers,  88-92 

of  tank  bottoms,  Imhoff,  419,  423 

sedimentation  tank,  404 
Skewback,  204 
Sludge.    Chap.  XX,  495-505 

activated.  Chap.  XVIII,  465- 
479.  See  also  under  Activated 
sludge. 

analyses,  414,  467,  468,  485,  496 
characteristics,  495 
definition,  495 

digestion  tanks,  427-430,  497 
disposal  methods,  495 
drying,  497-505 
acid  flotation,  503 
beds,  498,  500 
centrifuge,  501-502 
heat,  502,  503 
press,  500-501 
thickeners,  504,  505 
fertilizing  value,  470,  495,  497 


Sludge,  filters,  498-500 

lagooning,  495,  496 

measurement,  427 

press,  500,  501 

sedimentation,  401 

septic  analysis,  434 

treatment  methods,  495 
Soaps,  357 
Soil,  bearing  value,  125 

stack,  definition,  7 
Solids  in  sewage,  356-368 
Special  assessment,  15,  16 
Specifications.    Chap.  X,  211-232 

general,  219-229 

special,  230 

technical,  229,  230 
Spiling.    See  Piles. 
Spirillum,  362 
Spores,  363 
Springing  line,  204 
Sprinkling  filter.    See  Trickling  filter 
Square  sewer  section,  68,  69 
Stability,  relative,  359-361 
Stagnant  water,  374 
Stakes,  contractor  to  provide,  221 

where  driven,  281,  282 
Stationing,  92 
Stay  bracing,  270 
Steam  boilers,  147-150 
Steam,  consumption  by,  pumps,  144 

145 

turbines,  144,  147 
engines,  144,  145 

pumping  engines,  142-146 

pumps.    See  Pumps,  steam. 

shovels,  246,  252-254 

turbines,  146,  147 
Stearin,  366 
Steel,  forms.     See  Forms,  steel. 

pipe,  164,  191,  192 
design,  195-197 
specifications,  191 

reinforcement    for   concrete,    191, 
326-327 

sheet  piling,  252,  280,  281 
Stench,  historic  in  London,  4 
Sterilization.    See  Disinfection. 
Storm,  sewage,  definition,  7,  352 


530 


INDEX 


Storm,  sewer  system  design,  93-98 

water,  quantity,  40-50 
Storms,  extent  and  intensity,  50 
Stream    pollution,    regulation,    380, 

381 

Streams,  self-purification,  373-376 
Street,  inlet.     See  Inlets. 

wash,  definition,  352 
Stresses,  in  buried  pipe,  198-204 

in  circular  ring,  194,  202-204 
Sub-main,  defined,  7 
Subsurface  surveys,  18-20 
Suction  for  centrifugal  pump,  141 
Sulphur  and  sand  joint  compound, 

309 

Sunday  work,  221 
Surface,  elevation,  92 

of  ground,  character,  44-46 

profile,  88 

water,  7,  352 

Surveys,  underground,  18-20 
Suspended  matter,  357 

Talbot's  run-off  formula,  49 
Tamping,  backfilling,  328-331 
Tannery  wastes,  disinfection,  491 
Taxation,  general,  16,  17 
Taylor  nozzles,  444,  445 
Temperature  of  sewage,  353 
Templates,  brick  sewers,  312 
Thawing  dynamite,  301,  302 
Tide  gate,  122 
Timbering  tunnels,  286-288 
Timber,  strength  of,  277 
Time   of   concentration,  41-43,   95- 

97 
Tools,  for  cleaning  sewers,  337-341 

excavating,  242,  246 
Tower  cableways,  252 
Trade  wastes.    See  Industrial  wastes 
Traps,  in  catch-basins,  107 

grease,  gasoline,  and  oil,  108,  109 

in  street  inlets,  104,  105 
Travis  tank,  427,  428 
Tremie,  187,  188 
Tree  roots,  333,  340 
Trench,  backfilling,  328-331 

blasting  in,  244,  269 


Trench,  bottom,  shape  of,  241, 304, 311 

breaking  surface,  243,  244 

drainage,  256-263 

excavating,  by  hand,  242-245 
machine,  244-256 

guarding  and  lighting,  221 

layout  of  tasks,  243 

length  of  open,  241,  248 

line  and  grade,  281-284 

location,  243,  281 

opening,  243,  244 

pumps,  256-263 

sheeting,  270-280 

width,  240,  241,  246 
Trestle  excavators,  250,  251 
Trickling  filter,  437-452 

advantages,  438,  439 

covers  for,  451 

depth,  441,  442 

description,  437,  438 

dimensions,  442 

distribution  of  sewage,  442-451 

dosing  siphon,  446-451 

dosing  tank,  446-451 

head  lost,  438 

insects,  438 

material,  441 

nozzles,  442-451 
layout,  447-451 

odors,  438,  439 

operation,  441 

rate,  441 

results,  439,  440 

siphon  size,  449-451 

underdrainage,  451,  452 

unloading,  431,  437 
Tripod  drill,  265 
Triton,  295 

Troubles  with  sewers,  causes,  333 
Trumpet  arch,  121 
Trunk  sewer,  defined,  7 
Tunnels,  283-294 

backfilling,  331 

breast  boards,  288 

brick  invert,  313 

compressed  air  in,  292-294 

concrete  construction,  320,  321 

depth  of  cover,  284 


INDEX 


531 


Tunnels,  line  and  grade  in,  283 

machines,  290 

rock,  290-292 

shafts,  284-286 

shield,  288-290 

timbering,  284-288 

ventilation,  291,  292 
Turbidity  of  sewage,  353 
Turbine,  for  cleaning  sewers,  340 

pumps,  130,  132 

steam,  146,  147 
Typhoid  fever,  364 

U-shaped  sewer  section,  67,  69,  71 
Underdrains  for.  sewers,  126 

trickling  filters,  451,  452 
Underground  surveys,  18-20 
Unexpected  situations,  235 
Uniformity  coefficient  of  sand,  456 
Unloading  of  filters,  431,  437 
Urea,  367 

Valuation  of  sewers,  332,  348-351 
Velocities,  depositing,  395-397 
distribution  of,  51 
flow  in  sewers,  90 
over  surface  of  ground,  42 
limiting    for    sedimentation,    396, 

397 

limiting  in  sewers,  396,  397 
principles  of  flow  in  sewers,  51 
transporting,  396  , 
Ventilation,  air  pressures,  291 
compressed  air,  292-294 
pipes,  291 


Ventilation,    of   sewers,    102,    103, 

335 

tunnel,  291 

Vertical  sheeting,  270-274 
Vitrified  clay.     See  Clay  vitrified. 
Volatile  matter  in  sewage,  357 
Volute  pumps,  130,  132,  154 
Vouissoir  arch  analysis,  204 

Wakefield  piling,  273 
Wales,  288 

Waste  pipe,  defined,  7 
Wastes.     See  Industrial  wastes. 
Water  consumption,  31-33 

flow  of,  51-77 

rate  of  steam  engines,  144,  145 

supply  and  sewage  flow,  31-33 
Watershed.     See  Drainage  area. 
Weight,  of  backfill,  199 

of  building  material,  201 

of  moving  loads,  200,  202 
Well,  hole,  101 

points,  262,  263 
Wheel  excavator,  246-250 
\V^ing  screen,  384 
Wood,  forms.     See  Forms. 

pipe,  materials,  164,  165,  190,  192 

193 
design,  197,  198 

working  strength  of,  277 
Work,  extra,  227 

preliminary  to  design,  9 

Sunday,  night,  and  holiday,  221 
Workmen,  competent,  227 

dishonesty,  233,  234 


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